There could be a lot of reasons you are not having sex as often. The first thing that comes to mind is your wife might be totally exhausted with two children being a full time mom and you don't say if she works? If she does, she is doing double duty working and taking care of the children as well as the house. I would suggest she have a complete physical to make sure nothing physically is going on with her. Have you sat down and talked this out? To answer your question, I would say no it is not normal, but you have many questions that need to be answered before anyone can make a blanket comment. She just might be totally exhausted after a full day. If everything is physically fine with her maybe a week end away once in a while or take the children to grandma's house for the week end to give yourselves some time alone.