It depends on the age of the child. 

A mother with a baby under a year will be lucky to be willing/ wanting to have sex once per month…that is AFTER the two to six month required abstinence from the time of giving birth.  Mom’s who are breast feeding produce minute quantities of estrogen plus she has oxytocin coursing through her veins–causing a rise in sex hormone binding globulin (SHBG), which binds testosterone to the cells and makes it unavailable for things like libido, genital sensation and general energy.  For as long as she breast feeds, sex may not be the best experience for her…for some women it is downright painful.

Unfortunately, sleep deprived parents start developing poor intimacy habits which segues into sporatic sex once the child becomes less dependent.  Probably why pop culture espouses sleep as the new sex.

As for statistics on “how much sex is normal with kids” it is all over the map.  There haven’t been any scientific, peer reviewed research on the topic that I know of-only ad-hoc surveys which I believe too many people embellish.  If you go with those, most say couples with kids have sex once per week.  From the moms I’ve spoken to with small children it is more like once a month (or once every two months).

Once the child becomes a teenager it frees up a lot of time for the parents.  HOWEVER, they have had twelve plus years of building poor intimacy habits and don’t know how to get it back together.  So it usually starts to drop off even more to sex once every couple months.