I know that I am going to start a controversy here. Everything that I am about to relate comes from actual events that have happened to customers in my store. Riding children that small is a really bad idea. I know it is done in third world countries, but children get hurt. I have seen several children with burns and other injuries from this practice. With 2 children to hold on to you are really asking for trouble. One child slipped from Dad's hands and he ran over her with the rear wheel. She walked again in a year, sort of. Another got his arm stuck between the head pipe and the engine when he fell. The arm will never re-grow the burned off muscle.
I've seen a boy with part of his foot abraded off from getting it stuck between the swing arm and the wheel. I have seen kids in a body cast from the parent trying to catch them as they fell off the tank, then losing control and dropping the bike on the kid.
Yes, most of the time every thing is OK. I know it's been done a million times, but I've seen too many one million and one times. It's not worth it. Not for any kid. Not for my kid. I mentioned this in another posting: WAIT until the child is big enough, strong enough and will listen well enough to ride properly with proper gear.
How ever much fun your bike is, it is not a carnival ride. And life is not America's Funniest Home Video. Some times they don't get back up.