Is she in public school? How often is she wetting? If you send a change of clothes and she can change herself this should not be a problem. If she can not change her own clothes after she wets the Kindergarten teacher may not be allowed to change her due to school district rules.
Find out if the school lets the children have regular 'potty' breaks , sometimes they dont and children do not like to ask, I agree it is not a reason to dismiss her from a school it would be good if you protested about it and asked if the teachers tell the kids to use the bathroom .
My 4 year old wets her pants at school sometimes, and she also has a foul odor whenever she wets the bed at night. They are saying they want to kick her out of kindergarten because of this problem. I don't think that is fair. she is just 4!
Milk is a food, it would be better if he drank more water and some milk, it sounds as if he may be dehydrated is his urine dark yellow rather than pale yellow, ususally that is a sign they arent drinking enough water especially in the summer, askt he teacher at school to make sure he gets potty breaks, sometimes they forget small children need reminding when to go,it is hard to get children to eat the best veggies I know, try him on Peas and Beans they seem to be accepted better and are fgood fibre, what about fruit , bananas are great and full of everything good, apples, pieces of cheese, small cut up bits of chicken, fish sticks,take him shopping let him enjoy choosing some nice food, Cereal in the morning ,yoghorts, My guess is he drinks milk before bed , so cut that out well before bedtime let him have some water, that may help with the night time control. Check he is happy at school and is doing well at home .Good Luck