The youngsters are very lucky to have you to take better care of them. Here's a few things that helped ease the transition for us, and that work with the occasional visitor whose mind is blown by the fact that we don't have meat or cheese in our house.
-"So Delicious" brand products, including cultured coconut milk "yogurt" and coconut milk "ice cream." Don't mention that it isn't real ice cream and yogurt, and I bet they won't notice.
-"Hot dogs" and "chicken nuggets." Oh dear. Yes, the dreaded meat substitutes. Just for the transition. I do this occasionally for visitors, because my son has a couple friends who refuse to eat other things. So far, the visitors have not noticed that the "chicken nuggets" are not chicken. Of course, these foods are NOT really healthy... but according to the Seventh Day Adventist studies (according to Dr. Barnard, they eat a lot of meat substitutes), they aren't as bad as real meat. I do try to avoid isolated soy protein.
-"Daya" cheese. It is not cheese, and it is made with palm oil. Another "vegan" junk food. But if a coating of Daya works to persuade a child to *try* broccoli for the first time, and the amount can gradually be reduced over time, I wouldn't feel too guilty about it.
-"Fries" are easy... the "Happy Herbivore" cookbook has some great baked fries with very minimal oil and nice seasoning. She also has some good "gravy" recipes made with nutritional yeast, and also some nice "cheesy" sauces for veggies or pasta with no cheese. There is some fat added, but it is minimal quantities.
-Wait for a time when the children are *REALLY hungry*... then set out a tray of raw vegetables and fruits (carrots, cucumber slices, apple slices, orange slices, peppers etc.) with no dip. Don't ask if they want it... just put it out.
-A few other snacks have always been cheerfully received even by my child's most finicky friends: strawberries, grapes, watermelon, and fresh popcorn.
-Start with white rice, couscous, or ordinary pasta, and gradually, over time, mix in brown rice and other whole grain products.
-Only plain water to drink? Make sure it is served in a fancy glass, with a slice of lemon and ice. Buy some fun straws. Get some cheap fancy glasses at a discount store, or glasses with pretty pictures. If truly desperate, go to diluted fruit juice with no sugar added. Mix fruit juice with sparkling water for a few bubbles.
The psychology of all of this will be tricky. For children who have not been getting the emotional support they need, and who may have trouble trusting adults, I would hesitate to *command* or *insist* that the child eat anything he/she does not want to eat, or punish the child for refusing to eat. Commanding "eat this or else" might well get results along the lines of ... child eats, and then makes himself throw up later; child *still* refuses to eat even when punished, and dinner becomes a screaming match; child eats, but sneaks sweets and "bad" foods at every opportunity and hoards it. Here's a few things to help them make the change without threatening their sense of control:
1) Ask for their help deciding the menu, within limits (the cook is not running a restaurant): "Do you want to try broccoli or cauliflower?"...
2) Ask for their help in cooking.
3) Put enough "easy" dishes on the menu to give the child some choice: My son is reluctant to eat lentils, so when I make lentils for the grownups I make sure I have plain canned chick peas, green peas, or even mixed nuts available to fill him up.
4) When you go out to eat in a restaurant, let them order what they like, so long as it is not an immediate health issue (my son is allowed to order meat in restaurants, but we steer him away from dairy, because he reacts to it).
5) Avoid lectures... lead by example. Instead of a big stern announcement, "in this house, we eat only plants," just serve the meals, and keep it low key, and see what happens. If they ask, "Sorry, there are no hot dogs. There's just what you see."
7) If this is in line with your views, it might not hurt to mention that the plant-based diet is more consistent with wanting to be kind to animals. Just as light conversation. This has been helpful with my son, who is tender-hearted... it might *not* be helpful at all for children who are on the verge of shutting down emotionally, and anything that would feel to them like emotional manipulation should be avoided.
If you have any outdoor space at all, in the right season, have the children plant some food veggies--lettuce on the balcony in a container if you don't have a yard.
However, one doesn't want to compromise on *everything.* Here is where we draw the line in our house:
1) No meat, dairy, chips, soda, candy, SAD junk etc. is available. It is not on the grocery list.
2) Our son must taste *one bite* of foods that he says he does not like or is afraid to try. It can be a small bite, and he does not have to swallow it. Exception: we do not make him taste spicy food, because he seems to be EXTREMELY sensitive to it.
3) 5 bites of some kind of breakfast must be eaten. It can be peanut butter on a few healthy crackers. But going without breakfast is not an option.
4) Except at breakfast, if all of the choices we offer are rejected, the kitchen is then closed. If the child is hungry, well, he'll have to wait until snack time.
5) The cook decides the menu. No meals are made to order.
I would recommend taking about a week to see where the children are. Sit down on the floor and play with them to build trust. If they are still playing with toys and haven't been traumatized so much that they are trying to act like mini adults, try toy vegetables (playmobil?) and a toy garden. Maybe the toys can have a hospital, and the little toy doctor can suggest "oh... you have a bad cough. Maybe you should eat vegetables and fruit!" Watch them at mealtimes. See whether there is *any* healthy food they will eat, and start to build on that...
All the best to your newly expanded family!