Based on all other experiences, your 1.5 year old would not require a ticket at all. He/she would just go with you like a normal park entry for someone under 3...
I almost booked this today, and you just select them as a party member (i was booking for 4 myself, DH, DD7, DD1) and then when you select whos going and go to check out it shows that there is no charge for the under 3 person in the group.