What type of legal action can I take against my 5 yr olds summer program after they left her at the pool?

You cannot sue since you were not damaged but you can file a complaint with the local and state licensing agencies and you should find out exactly what the child to adult ration is for field trips. Most of the centers I am aware of do not take the kids on field trips unless they have parents to help make up the chaperone quota and that seems to be around 3 to 5 kids per adult. Get your facts, write a letter of complaint to the owner and the director (often not the same person) of the facility) with a copy to the state board. They also should have a buddy system so that each child has another child who holds their hand and when head counts are being taken the pairs of children hold their hands in the air. That way if there is one kid alone it is easy to notice it and start searching befor the bus leaves.