Why are they doing this? Because they know that the Israelis will not target an ambulance, even though Hamas has used ambulances to carry weapons in the past. This belies their claims, however eagerly repeated in the mainstream media, that Israel is indiscriminately or even purposely targeting civilians.
“Gaza Terrorists Using Ambulances and Kids to Move Around Safely,” Jewish Press, July 18, 2014 (thanks to Pamela Geller):
A senior IDF officer reports that terrorists in Gaza are transporting themselves around Gaza via ambulances with children in them, to avoid being targeted by the IDF.