i brought my girls to the YMCA for a long time, and the local community college by me also has swim lessons. Now, they are at a point where they need 1-on-1 lessons (weren't getting much out of the group lessons... too much sitting around while other kids were having their turn), so I've been working on it with them myself, and they've made great improvements in a short time!!! My friend has had great luck with the YMCA by her... it really depends on the instructor.
this is the swim float they used at the YMCA... i bought one for my girls. You take one section off as the child progresses with their swimming. Depending on their swim level, you may be able to buy one and split the floats between two kids (i did that... just bought belt material and a snap at Jo-Ann Fabrics).
http://www.amazon.com/Water-Gear-Float-Progressive-Orange...