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Long johns suit younger kids 0.5 to 5 years x2. Long tails suit 6 - 10 year old x2.
I wouldn't put a 3 or 4 year old on a long tail without being in a seat with seatbelts, they either try and climb out, can't hold themselves in, or most commonly, fall asleep then slump over the rail or can fall out of the gap. When they are older/taller the long tails work well. Can't as easily get that nice washing machine you found on the side of the street home on a long tail though ;)
Bullitt is short in the leg beginning around 5. Raising the seat will help. If you have just 1 kid then width is not a problem but length leg space maybe. I think for 1 kid that age seat on the rear with a cage around it (eg tern short haul, quick haul, gsd with a clubhouse) will give you from now untill they refuse to get on.