I was basing my calculation on your daughter being under 2. So Gatwick to Stornoway works out as ?680 for flights on 21st July -28th July for the three of you via Inverness.

The cheapest possibility is to fly from Stansted to Glasgow Pestwick (from ?8 per person) and then fly from Glasgow international to Stornoway (from ?32 per person). Obviously the connection is a pain, but if you are staying overnight in Glasgow then it could work well.

I do all of my calculations on http://www.skyscanner.net which also has a very helpful route map http://www.skyscanner.net/route-map