I took my 7 yr old for a lengthy-ish t-a-b ride this morning, 1st time since fall (a few very short outings, basically me running it up to school to pick him up at the end of the day if he didn't bike there himself), and I have to say I felt pretty wobbly, kept wishing my handlebars were wider to get a bit more leverage. I seldom felt comfortable taking a hand off the bar for a traffic signal. I don't even think he was doing his usual wobble, bend, stretch all to one side schtick today like he will on a longer trip if he gets bored.
Oh yes, as he gets bigger, in addition to being a bigger load, his seatpost goes up so the center of gravity raises that way too. This is an adams t-a-b that clamps to the seatpost, not like a burley that clamps to a rack.
I don't know if this is just a fact of life of a growing kid and I should have made him ride his own bike to the dentist this morning after all.