i have looked for them too and didn't find any. i believe that they only have the adult and child sizes.
yes, while in the stroller the child size was fine as it was long enough to hand over her and the stroller and keep her dry without dragging all over the ground and getting caught in the wheels.
we use one like that for when she was in the stroller and just left it on the stroller when we had to leave it to go in someplace. (take along a few clothes pins to pin the poncho to the stroller in a few places to keep the wind from blowing it off and you come back to a wet seat.)
for the toddler to wear, we brought along a toddler sized rain coat. (one day when we forgot the rain coat at the resort, we bought a child poncho and borrowed a pair of scissors and cut it off quite a bit. not ideal and mickey got hacked in half, but it worked in a pinch.)