Children of all ages are usually permitted to fly on helicopter tours, providing they're occupied by a parent or carer as to make sure there belts are kept on and can be looked after.
Toddlers over 2 years on most flights will need to be sat on there own seat, which needs to be paid for with the price of an adult ticket.
If the child is under 24 months, then they can still fly but need to be fastened on the parents lap. Under certain circumstances a young child can fly on the front passenger seat, but rarely are permitted to.
Never will a group of children be allowed to fly without supervision of an adult.