Inverness is a great place to live. Good quality of life and loads of things for children and teenagers to enjoy, including great activities on offer relating to the outdoors and traditional music etc. Yes,some things are restricted compared to what might be available in a city but there are other things which make up for this if you are looking for a different way of life.
State schools are generally good and usually within walking distance (though there are no private schools within easy travelling distance).Although there is now a university here, most young people still need to leave home to study and the chances of being able to come back north afterwards to work in a graduate level job are still very restricted.
It doesn't have the same range of shops as Edinburgh or Glasgow etc (though a lot of the high street chains are represented) and the A9 is a bit of a pain (or at least will be until dualling is complete) but there is always internet shopping (even though some companies treat Inverness like the Outer Hebrides in terms of postage costs). We have a great theatre/arts centre, good leisure centre (and private alternatives), terrific outdoor facilities, and loads of beautiful places to visit within easy reach.
Living outside the town might be more difficult from a practical point of view as rural public transport is limited, taxi-ing kids around could be quite time-consuming, and most country children are bussed in to secondary schools so they have early starts and are later getting home. In bad winters you also have to consider icy roads etc. (But, by and large, the weather is OK - not as warm in summer as further south but the winters are generally milder and my relatives in the north of England often have worse weather than we have. I also don't recognise some of the descriptions above!)
There have been a lot of cutbacks in public sector jobs recently and opportunities for good jobs in the private sector are also limited so it would be worth checking the employment situation out very carefully (although it sounds like you are already doing this).
I would guess you would find property good value if you are moving from London.
The post from Jacks11 makes some very good points.
On thing worth pointing out is that although Inverness is not the back of beyond, the distance from family and friends in England can be a real issue both time- and cost-wise.
I don't know anyone with an au pair so can't comment on that but I would think it would be an attractive location for many au pairs and there is also a good selection of other childcare options.
It is horses for courses though, and living somewhere is always going to be different from seeing it as a holidaymaker. If you try to live a London life here and measure things like shopping, job opportunities, and cultural activities in London terms you will be definitely be disappointed, but if you are willing to adjust and embrace this area's own strengths and opportunities, it has a lot going for it.
Good luck whatever you decide.