Its perfectly doable, if your 5 yr old is fit and willing. Just buy a airport day ticket, sit in the S8 or S1 and go downton, which is right above the 'Marieplatz' station.

If the child is not really fit, you can explore the airport itself: https://www.munich-airport.com/at-the-airport-260393

If you want to just sleep, the on site Hilton sells dayrooms and two hour passes to their pool/spa area (kids welcome): http://www3.hilton.com/en/hotels/bav...spa/index.html
http://www3.hilton.com/en/hotels/bav.../100074547.htm