have taken my son on pirates at 7, 15, 17, and 19 months. totally fine. there's a slight dip but it's not like the mother won't be holding the baby. in fact, the last time he was on it, my son loved that ride. i have a little pirate in the making! AAARRRRR!!!
One thing with baby carriers... I once witnessed a lady take a nasty spill using one in the WS. The baby ended up fine, but mom did not. DH and I were thinking the way she fell she was trying to rotect the baby, and as such, did not have time to brace herself. Generally, I just think a stroller is safer, but maybe that's just me.
Back to OT - I think taking a small child on PotC is fine.