Things I can think of that may be open when you're here:
Brodie Castle, nr Forres. National Trust for Scotland. 16th-century tower house with art and antiques. Castle open 1 Apr to 25 Oct varying times/ days. Grounds open all year, daily, with child-friendly duck pond.
Dunrobin Castle, nr Golspie. About 1 hour from Inverness. Fairytale style castle and victorian grounds. Open April to October.
Falls of Shin, as mentioned above. Although a pal of mine re-named it the Falls of Sh*t, following a visit. I've not been myself so can't comment.
Fort George, Ardersier. Open all year. Massive fort with artillery fortifications; regimental museum of the Queen’s Own Highlanders; collection of 18th-century arms and military equipment; barrack rooms recreating soldiers’ living conditions in olden times etc. Prob one for little boys, rather than little girls?
Plodda Falls, nr Tomich. 2x circular, shortish waymarked walks to Plodda falls from the car park. From the A831, from either Beauly or Drumnadrochit, take the unclassified road signposted to Tomich. Go through Tomich and continue along the bumpy forest track. Plodda falls carpark is 3.5 miles along this road.
Jacobite Cruises on Loch Ness. I can recommend the 'Freedom' tour, which consists of 30mins boat trip on the loch to Castle Urquhart, an hour to look around this pretty impressive castle, then 30mins boat trip back. Operates all year round.
Culbin Forest, near Forres. Forestry Commission plantation on former sand dunes. Easy gradient well surfaced paths with interest points along the way. Lookout tower in the forest. Good if you can hire/ borrow a couple of bikes with child seats (don't know if this is feasible).
If you find yourself on the Black Ilse, you could try spotting dolphins at low tide at Chanonry point, nr Fortrose. Fortrose cathedral is worth a look too (its actually a ruined church).
Tain takes about 5 mins to look around IMHO. Although the Glenmorangie distillery nearby is excellent (you'll have to lose the kids first :-)).