So, I'm planning our wedding and thinking about the number of guests who will be at our reception. We are allowing children since we are having it in a location where most of our family and friends will be coming from out of town, and many of them have children. It wouldn't be fair to ask them to make alternative arrangements for their kids when traveling in from out of town. So, most of the kids will be under 12, with about 10 children aged 3 and under (if all of these people I'm thinking of will be able to attend- I just realized how many young children will be there- holy cow).
Anyway, my question is, is it reasonable to expect kids aged 3 and under have food provided for them by their parents? I'm thinking of doing something easy for the other childrens' meals, maybe a kids buffet of something like mac & cheese and chicken nuggets, foods that are general crowd-pleasers with kids. Any tips on how y'all handled this at your weddings would be great. Thanks!

Re: Age cutoff for catered meals for kids?
I know many little ones between the ages of 18 months and 3 years who will eat chicken nuggets and mac and cheese. I don't think you should assume parents of kids ages 3 and under will provide their own food.
It sounds like youre having a buffet, so i assume you wont have food info/choices on your RSVP card, correct? You might just have a line that says something like "Kids' meal needed? _____"
Bad idea. Children, especially 3 and under, should be seated with their parents.
I have to say that I would have never thought of bringing food for my 2 year old because at that age they are eating table food.
As a mom of 5 I can't encourage you enough to NOT have a kids table. In a setting such as a wedding, parents want those kids on their best behavior with their best manners. Seat them away from mom and dad and it might not be pretty at all. Not to mention the oldest kids at the kids table end up more like babysitters. You really want the children with their parents or you could have a real mess on your hands.
You sound like you are trying to be thoughtful in all of this but I think you are putting things on your plate that don't need to be there. Most parents will not leave their child with an unknown sitter, and a lot of children might flip out being with a stranger.
I think you should plan for kids meals for ages 10 and under, seat kids with their parents and let the rest of it go.
One major caveat - you may have some 10 year olds who won't be filled by a kids meal. when my son was 10 he was well over 5 feet tall (we grow em big and hearty). He quit eating kids meals when he was about 7 because there just wasn't enough food in them. As one pp mentioned, you might want to call the parents and ask them if the child would want a kids meal or an adult meal.
And when exactly are out-of-town parents supposed to stop at a store or a fast food place between the wedding and reception to acquire food for their kids, let alone actually cook it? Your venue is very likely not going to allow them to bring in outside food and drinks due to local health codes. And where in a hotel room are the parents supposed to store and cook it?
Logistically, this wouldn't work even if it were polite, which it is not.
So I think you need to budget for meals for all your guests, not just adults and kids over a certain age. If the kids don't eat them, that's their problem, not yours.
I would not expect parents to bring food for kids under three. Snacks are fine--I always travel with several--but to bring a full-blown meal is inconvenient, and honestly, not the best hosting. (E.g. "We'll feed some members of your family, but not all of them.") For a full meal, you'd have to think of something that you could keep at the appropriate temperature, may not be able to reheat, etc.--it's a logistical challenge that a good host shouldn't push off onto their guests. And kids aren't oblivious--if they see their parents or older sibling chowing down on chicken and pasta and they have a PB&J in a baggy, they're going to want what everyone else is eating.
TL;DR: feed the kids
I know you're still throwing around ideas so here is what we did. Nothing.
We had a buffet and the same meal was provided to everyone who attended. There were chicken, beef, and veg choices and a variety of people pleasing types of side dishes too (so basically if we had a picky eater there should've been enough options that they could fill up on SOMETHING even if it was just bread or starches). The only difference between adult meals/kids meals was our caterer charged less (or nothing) based on the age of the kid. All children were seated with their parents/caregivers and as far as I know everyone was happy.
My SO would love to be offered a kids menu. Meanwhile, my two, almost three, year old nephew happily ate half of my bowl of tom kha gai, several bites of his mom's green papaya salad, and put a pretty large dent in his father's panang curry. I'm not even sure the kid knows what a chicken nugget is.