If you only go Motel-one/Allianz and back a single/return would be cheaper, any add-on and a day-pass would be cheaper - also depends on age of the kids.
I think your best option is a group-day-ticket (you only need 'inner district') http://www.mvv-muenchen.de/en/ticket...ket/index.html - peace of mind and all that.

The app seems to be unable to purchase group-day-tickets, weird.