Hi A.,
I live in Santee as well and I just love it here. East County gets a bad rap but Santee has become quite a great community in the past few years. Now my daughter is only four, so I'm not yet familiar with things to do for the older ones. I take her to the Santee YMCA for gymnastics, but they do a lot of other activities there as well.
Santee is a great place for sports if your son's are into that. Go to the Santee web site and check out all of the sports and activities that they can sign up for.
http://www.ci.santee.ca.us/
There is always a lot going on in the summer months at Santee Lakes and at the Trolley Square Shopping Center where they do little concerts. If your family is the outdoor type, mission trails has great hiking and biking trails. For your older son, both Santana and West Hills are great High Schools. Santana has always had a good sports program.
It is scarry moving to a new area when you have lived in one place for most of your life. We moved out to Flagstaff for a few years. It was quite an adjustment, but it was a great experience. It will take some time, but I'm sure you will like living in Santee.