Is your baby walking yet? Get a bar in his hands! No, I'm just kidding, kind of...but in all seriousness, it is so benofficial to start children young in some sort of programming. I'm not saying call Glenn Pendlay up and schedule a session tomorrow for your 3 year old but getting him or her into a gymnastics class or even some sort of kids sports program to teach body awareness is spectacular. If you work on engraining proper technique even in just the squat movements your child will remember these movements as he or she gets older, even if there's a long period of time that they don't use it. "It's like riding a bike," is the perfect coined phrase. This is what is referred to as muscle memory.
In addition to laying a good foundation for a more fundamentally sound child in regards to sports, weightlifting, etc.; you are also doing your child a huge favor by getting him or her off the couch and moving. Living in such a fast paced world it is easy to get caught up in the quick and easy. Going through a drive through to order $1.00 cheeseburgers is one, cheaper than fixing a dinner at home and two, convenient and easy. More and more kids, and even people in general are suffering from obesity problems. In most circumstances these are probably the kids camped out on a bean bag playing video games and eating Cheetos on the prettiest summer day. Start your kids young on making proper eating habits as well. I'm not suggesting you shouldn't let "kids be kids," but a candy bar as a treat or reward is much different than allowing your kid to have one in substitution to breakfast. Pay attention to snacking habits and grazing times. Help your child make good decisions and talk to them about the consequences the bad ones have.
You are responsible for your child's health, don't let them get started off on the wrong foot just because it's easier for you. Set good habits now because bad ones will only start to form in the future. Get them in some sort of program and get them on the right track for eating. Even if it's not fun right now, they will thank you in the future.