Why is it easier for a baby/toddler to wear a bikini than just the swim bottoms? Mine had nappy type bikini bottoms and didn't worry about covering their tops at all and plenty of littlies on the beach were like that- and some naked ones as well.
For older girls, I love the tankini styles, very sensible, I don't want my 9/10 year old in a frilly bikini, it sends a weird message out and invites people to think she's older than she is.