Pleasure Island is free to enter for any age. You only pay if you want to enter the nightclubs. You have to be 21 or over to enter the BET Soundstage and Mannequins. For most others, you need to be 18 or over.
PI is really intended to be an adult nighttime entertainment venue, although you will see some children there, particularly earlier in the evening. If you want to take them, your best best is probably the Adventurers Club.
If you do go, don't forget to take some ID if you want to drink. They will card anyone who looks under 30 (or even older in my experience) and you won't get one of the requisite 'bracelets', which tells the bar staff that they can serve you, without it.