Lots of good stuff has been said so far. A few things:
1. You said, "The Happy Meal idea is cute, but I’ll have to see if the reception facility will allow outside food (our wedding is at a vineyard that has on site catering)." If you find that it does not allow outside food, you must offer food to ALL of the kids. Otherwise, how will they eat?
2. Yes, as Theresainpa said, you might be surprised that a two year old can eat well. And there might be a 7 year old, who doesn’t. Children between 1-2 can probably eat most of what is offered on a kids menu. Children under 1 can probably eat some things, depending on what is offered, and kids under six months are probably mostly on jar food.
3. Give parents some credit. I have children. Chances are I wouldn’t bring them, in the first place. But for those who do, I think they know their children well enough to know if they are going to eat the food or not. If they are infants, they are probably going to bring baby food, not order a meal for a child who is too young to eat it.
4. If you can bring in McD’s, I would offer that as an option for all kids. The older ones might like it better too. (If this is an option with your venue…. IDK.) But if you can just get them to bend for the littlest ones, I’d say leave the parents the option of ordering from the kids’ menu with the venue, or ordering from McD’s. And if you do offer McD’s, make sure someone is in charge of paying and delivering it to the reception.
5. I don’t think you should exclude offering dinner to some guests based on a presumption that they will or won’t eat it. If you are inviting them, you need to feed them. If you can offer McD’s to young kids, they’ll probably enjoy that. However, you might be surprised that some two year old will eat their kiddie meal better than your cousin’s date eats her adult meal. If you are inviting an adult you know is picky, will you not offer her dinner because she is likely not to eat much? If you are hoping by not offering dinner to the little ones, people will leave their kids at home, consider simply not inviting them.