I feel such a fraud asking this as I live in Perthshire but we are working out what to do in our week off together which is mid August.
If the weather is good we plan on Blair Drummond, Edinburgh Zoo, the Deer Farm and possible Deep Sea World. We'll do picnics and be out all day however, we all know how rubbish the weather can be so we need a back up plan and I am thinking that if it is rubbish that we might go away but somewhere in Scotland to a nice hotel with pool, spa but also some things we can do with DS that will keep him entertained. A bit of damp weather won't bother us but the torential stuff will make our good weather plans fall into chaos but I have no idea what to plan or look at or even if there is a good family hotel with activities that don't involve being outside all day.
I haven't been with DS but Crieff Hydro is meant to have lots of childrens activities and two separate pools, one you have to pay for which is adults only. Or if its really grim here you could try heading further South, borders seem to have better weather,or apparently Dunbar is known for its sunny climes, we go to the East Links park there and its generally good weather. Or you have some great indoor activities in Perthshire. The leisure pool in Perth is very popular with our DS (5) as is Noahs indoor softplay.
How old is your DS? I second Crieff Hydro, it's great but might be pricey at that time of year. Noah's ark has bowling and golf driving range as well so lots to do if it's raining. Further out, the nuclear bunker over near Crail was a good day out. Dundee has sensation, leisure centre and the endeavour so you could base yourselves there and have plenty of rainy day things to do. I don't know a nice hotel in that area though. St Andrews has some small museums and the castle so would be good for a day out.
we went to dumfries earlier this year with in laws for couple nights, lots more to do than I expected, farm park, national trust castles etc.. the cocoa bean company chocolate factory, galloway ice cream which has play barn. We are just back from derby. crieff hydro is fab, kids love it but defo pricey in august unless you can try for sun night or thur night deal.. Peebles hydro in borders nice too but kids club from 3yrs. Went to inverness couple years ago and travelled around, stayed in kingsmill hotel, pool was nice, walking distance to town, play park acros road and good few things to see nearby... Aviemore has also been recommended to us by friends... Good luck Ps would you stay over falkirk way/stirling? Falkirk wheel quite cool too. Ayrshire also has good few things.