"..and I returned to doing normal things with kids my own age.." This is the author way of saying: "...and I returned to 'the thing that I was doing before' with kids my own age...", this makes the verb a noun. Its like has if the subject just carries out what he or she was doing before. Ex.: Nothing deflates impossibility like the doing of it.

the structure should be "to return + to + sth" so "doing" here is a gerund, which means a noun :)

In English, you can always "return to" an infinitive. It might seem like a violation of the way an infinitive is supposed to work, but it is actually a legitimate use. It means that someone was doing something (the infinitive), stopped doing it, then started doing it again. So if was reading a book, then stop for some reason, then start reading the book again, I can say that "I returned to reading the book." Below are some examples. I returned to painting. (I was painting, then I stopped, then I started again.) I returned to eating. (I was eating, then I stopped, then I started again.)