I'm on Atkins (for weight loss reasons, not medical), and I would be disappointed if I went to a wedding and found the only entree choices were chicken alfredo and tortellini. But big whoop -- I'm disappointed. Not the HC's problem. I wouldn't consider them rude for not providing a carb-free entree; this is my issue. I can eat the salad and maybe pick the chicken pieces out of the alfredo. Or declare a cheat day for myself and eat the pasta. Or pick something up on the way home.
I think the OP is safe in ordering half-price meals for guests under 7. Even if one or two 6- or 7-year-olds eat a lot, they'll probably be balanced out by an adult or two who doesn't eat much.
Definitely don't do separate plates for salad. Juggling two plates at a self-serve buffet is a recipe for disaster, especially if there are long dresses, high heels and/or children involved. And besides, if guests are required to put their salad on the same plate as their entree, they're less likely to take too much of the entree -- not enough room on the plate!