I am very confused about how to register and how much I need to pay.

If you have one child joining one level of camp only, then you just need to pay for 1-seat ticket.

If you have one child wanting to do both levels of the camp, the you need to buy a 2-seat ticket.

If you have two children doing the same level, then you just need to pay for 1-seat ticket.

If you have two children doing different level classes, then you need to pay 2-seat ticket.