Personally I wouldn't use Google maps to find campsites when its mission critical (i.e. travelling with an 11 and a 13 yr old) to do so. When I was travelling with my kids at that age (they're about twice that now) we always made sure we knew as much as possible about our next destination and often had them make the choice. That meant guide books back then as mobile internet wasn't very practical.

In your position now I'd have them do the research (either online or in books) while you were in one site and book it ahead. It's not unusual for popular sites to fill up on a daily basis by people just turning up at 5.00 / 6.00pm. The last thing you want with kids that age (i.e. old enough to know what's happening but not old enough to see alternatives) is to turn up at your site of choice and be told its full.

While there are thousands of campsites in France /Spain /Portugal they're not all the same (to say the least). A lot of the ones full of facilities that kids would love (on the Med coast for example) are not really campsites as such but more like holiday camps. Many of them are not really set up for one or two night transient stops but are aimed at the there for a fortnight market. My advice is to put some effort in beforehand (despite the internet I still turn to guide books) and find out whether they are happy to take overnighters.

At the other end of the spectrum (in France at least) are the municipal sites. Seemingly almost every village has its municipal campsite. In the UK the word municipal is most often linked with the word "dump" so a municipal campsite doesn't conjure up the best of visions but many of them are wonderful places for one or two nights - and busy. A few nights ago I just managed to get into the municipal site in Dijon before the full notice went up. Some, on the other hand, are, well, lets just say, basic. Again, it's having information about a place before you turn up. I used a 2011 Michelin campsites guide that I bought cheaply from my local Oxfam bookshop and it was fine (although I doubt it had 10% of the sites in France in it) but there are many available.

Re documents - passport(s), driving licence, reg V5 document, insurance cert (make sure it extends your UK cover and not just rock bottom basic eurocover), breakdown insurance. Take your MOT cert (if you have one) but tbh you'd be unlucky to be asked for it. You will need your passport to get into campsites as they'll want I.D. Also, european health insurance cards + medical cover insurance.

Tents - I don't know if you have tents but if you don't, get bigger than you think you need. The kids may be of the age where they may want their own little ones next to your big one. When you get to a campsite think very carefully about where you put it /them up. Not under trees (lightning strikes and dripping for hours when it rains) but shaded by them is ideal. Work out where the sun comes up in the morning and try to get the tent shaded for those couple of hours. There's nothing worse (for me anyway) than being burnt out of the tent at 5.00am as the rising sun hits it full on.