It depends. In CA we monthly (or bi-monthly) participated in 2 ongoing ministries. One was a special needs ministry to kids with special needs. My kids would play with them and we would supervise while we gave the parents some much needed respite.
The other thing we did was to take meals to the homeless shelter and sit and eat with them. It was an awesome ministry.
Here, we have done a variety of things. There is an inner-city ministry our church is a part of. We have gone to various events with them to help out, donate, etc... My kids even raised $150 on their own for 24 backpacks to donate to the back to school event! And then we went to help pass them out (and organize them, etc...)
My kids are young (9, 7 and 3), so we have to find things that allow them to participate.
Cubscouts does some charitable things like cleaning up the area, fixing fences, and donating toys and such at Christmas. We do that too, but since the boys are earning badges in the process, it isn't quite as much of a sense of ministry.
Does your church have things that would allow kids to help out? When we moved here I had to really search out some things and it took a while. I am still working on finding things.
Dawn