If you have young children and want to eat in a relaxed atmosphere there's plenty of choice in the U.S.: the vast majority of restaurants, and eateries are casually oriented and have children in mind, even babies. Obviously a lot depends on where you are staying -- if you're in Orlando then you will have too much choice. On the beach in Ocean City you'll have to walk a few blocks. Fast food restaurants such as McDonalds, Wendy's  and Burger King are everywhere.  If you are on a budget,  to find a buffet style restaurant, like Golden Coral, as these are usually overrun with kids and for a fixed price you get an all-you-can eat meal. Pizza, pasta,roast chicken, mashed potatoes, macaroni and cheese, steamed veggies, and a few others are pretty typical of any American buffet and will fill small tummies very easily. 

Kids are usually running around getting some exercise after that long drive, so if you want a peaceful meal in a child free zone, you've been warned. Shopping malls usually have a central Food Court surrounded by take-away food outlets - usually pizza, Chinese, subs (really big sandwich on a baguette like roll filled with deli meats)  and loads of good value meals. (Restrooms in malls are always clean to a high standard - so don't worry about little ones getting ketchup everywhere -- washing facilities are excellent).

 For sit-down restaurants, most American venues will accomodate children: the key to knowing if a restaurant is child friendly is usually the menu. If it is filled with foodstuffs that are expensive, has people out on the patio that are carrying BlackBerries and briefcases,  and seems to be geared far, far away from children's tastes it should probably be skipped. (No child is going to care for quail drenched in olive oil and garnished with Perigord truffles and fewer will want to wash it down with a 25 year old Burgundy.) In child friendly establishments the cutlery won't be child sized, but the policy of most managers is to offer slightly smaller flatware like dessert spoons and cake forks which are easier for little hands to wield (ask for this if it is not given outright.)  Most restaurants will provide a high chair for toddlers and babies or booster seats if your son cannot see over the table, and in some they will stow away a pushchair behind the counter if the restaurant isn't busy.