I am an Early Childhood Major and have done many observations in the area. I second the YMCA. It is the only center that is an accredited, licensed center with fully trained staff and teachers. The other gyms just hire whoever and do not adhere strictly to child to staff ratios and do not have a true interest in providing child care.. I had a friend who worked for Videorama's Gym and she said there were times where it was her alone with 20 toddlers!(The state limits are 4 toddlers to one adult) She was not certified to work in childcare, she was actually a desk clerk who had to take her turn in the child care room once a week. Oregon's child care regulations are the second lowest in the country, so that is pretty scary. So that is why it is especially important to find a nationally accredited center. I would go in person and observe the gym you are thinking of going to, and ask for credentials and accreditation from the NAEYC.(National Association for the Education of Young Children)I know it is only for a short time each week, but it is very important that your child be safe and properly engaged and cared for. Having a traumatizing experience two or three times a week is not ideal.
I know the assistant director of the YMCA in downtown Portland, and would highly recommend the center.(I took Early Childhood Ed courses with her in college)
Good luck in your search!