Oh boy can I commiserate too! I actually came here looking for answers to this exact question. I have 3 kids under 5 and the eldest is significantly underweight, always has been, as happens to get most of his calories from milk, yogurt and meat. I am following the Vegan lifestyle for myself which I love, but I can't see taking away my son's main sources of food. I have decided to allow him to have his favorite foods but I am spending a lot of money and time to make sure that the ONLY sources of his meat and dairy products are small, local farms that I have visited in person. That way at least I can attest to the fact that the animals have never been mistreated. I do a bulk buy a few times a year and store it in my freezer. I buy raw milk from a small local farm each week at the farmer's market. For dinner I serve my vegan food with a small serving of meat or dairy products as a side. I am hoping that by modeling this lifestyle myself and not forcing it on my kids that they will begin a slow transformation. This is the best solution I can come up with and still have a clean conscience. It is great to hear how others are doing it too.
I just got the Vegan Lunch Box, and it is a really great book. It has wonderful lunch ideas for kids and parents alike. It'd divided into receipes and then meal plans. However, it is as much as a hit or miss as anything else as far as picky eaters are concerned. My five year old was picky even before I started the Vegan lifestyle, was vegetarian for well over a decade, but now that I'm trying to remove his meat and dairy he's gotten even picker. He's not much into any food in it's natural form, but I've at least been able to sneak in meat and dairy substitutes into his diet. I know it's not as healthy as "real" food, but I figure it's better than processed meat and dairy. The trick is to not let into the fact that it's not actual meat and dairy. If he noticies a difference, I always tell him that the old brand I bought is no longer available and he's been really good at accepting it. I am really excited for the new book coming out, "The Complete Idiot's Guide to Vegan Eating for Kids". I'm interested to see what new insights it might provide. Good luck everyone!
You know what has worked for me, I have planted some small vegetables, Like tomatos, Peppers, Basil, things like that, I have my children help me to care for them. Also when I cook I let me 5 and 6 year old to help me with the cooking and for some reason that has given then more of an open mind to try things. I know making dinner may not be the easier or neatest but, it will plant a seed for them. They love it when I say, Jesse can you get me 5 tomatos, Maya can you get me a buch of basil, ok lets make a delish tomato salad. They will chance just hang in there, let them help you choose the vegetables and for sure let them help you with DInner. Good luck.