I think you would be best served to look more towards a bakfiets type cargo bike that puts the cargo up front and lower to the ground rather then a long-tail type bike especially since it's the front of the bike that is lengthened to accommodate the cargo instead of the rear of the bike with many models a kid's "half bike" trailer set-up will hook right on behind just like a regular bike.
If the kid(s) are in front of you instead of behind you can keep an eye on them better and since that design puts them down lower to the ground on a low slug deck or low tube with seat(s) you can balance heavier loads easier and their squirming around should makes less of a difference.
Some models have even been made with pedals for the kiddies up front (don't worry the rear handlebars for the adult driver in back are what control the front wheel and the kids handle bars are just fixed stoker bars). Such as the bikes in these pictures:
