Is it to be rainy all day?
Not sure where you are staying, but there is a swimming pool in the MacDonald centre that general public can use and also some of the hotels used to do a swim lunch in the hotel pools.
You could take the funicular to the top of the mountain.
Theres a climbing wall that maybe the adults would enjoy?
have a look at Rothiemurcus estates, they often have things such as a Jeep safari.
There is also a soft play/creche in the Hilton Coylumbridge for little people.
Or just embrac the rain and hire canoes/sail boats for the adults while the little ones poddle in the loch (at Loch Morlich)
The Highland Wildlife Park, The highland Folk Museum and the Landmark Centre are all good if you don't mind getting a little wet.
There is also a steam train sometimes up to Boat of Garten (I think).
The coylumbridge usually has evening entertainment on for kids at about 5/6pm, technically for hotel guests but If you had a drink in the bar I don't think it would be a problem - bar is quite expensive.
there is a little visitors centre between Loch Morlich and the Funicular with a cafe.
If I think of anything else I'll add to it.