Summer league swim gear is very minimal. It is best if each swimmer has a swim suit suitable for practicing and racing in. For girls this means a one piece suit with no ties or straps that may be loose. For boys this is typically a jammer or brief style suit that is well fitted.
Goggles are also a must! Please remember kids will need these each day at practice and it is best to have your own pair.
Towel and sun screen are good ideas too! Don't forget a drink for after practice.
Fins are used in practice for the 10 and under groups, it is not a required piece of equipment but is helpful for training and strengthening flutter kicks.
The team supplies kick boards when needed at practice.
This blog on summer league swim gear can be a handy reference for the brand new swim parent.