Pros
It depends on which aspect of the YMCA you work for. I was a part-time worker for the after-school program at the Anaheim YMCA. There is little training, it is more learn-as-you-go. We worked with kids as group leaders, aka instructional assistants, in the after-school program who were usually low-income students. Some of the kids are initially hard to deal with, especially if you don't have prior experience or you have a naive perspective, but this is because the kids are used to seeing a lot of adults and workers come and go. If you make it past the first month, then you will have no problem moving forward in this career. You will also become very attached to the children. And parents are very supportive. If you have a problem in dealing with a child, the parents will most likely be on your side. Coworkers and supervisors are also very helpful and understanding, especially to new employees. I would have worked there longer, but I had to move when I transferred to a university.
Cons
The only downside comes if you have no experience working with kids or if you have the idea that kids are innocent and nice. You have to remember that you are an authority figure and not the children's friend. That does not mean you have to be difficult. But when a child acts out, you have to consistently discipline them and be firm or else they will not take you seriously. I have seen a lot of workers leave because they weren't able to handle the stress of the environment. Some of these kids have other issues in their lives, so you the YMCA sets up regular orientation and training programs to help you deal with the children. The downsides aren't caused by the YMCA itself, but the nature of the work.