I know this is a really really stupid question, esp. as I am onto my 2nd child, but how much rolling on the floor stuff do they need? DS2 is 3 months & already rolling so I don't like to strap him in the buggy all the time. Unfortunately I often need to go out when he's not asleep (as when he is I grab 40 winks myself) but worry that he's then not getting enough exercise. I know they're constantly growing & everything so maybe they only need a few 10-minute bouts on the floor? Please enlighten this very stupid person...