JJ. because I work term time only as a nanny and my two are 14 and 15, soon to be 15 and 16, they are always away for the hols on school trips of some kind, my DH has also been working away for most of the week, I often fill in for families during the school holidays. I don't need to do it, but I enjoy working with kids, babies especially.
I have advertised my services on www.nannyjob.co.uk - I am a qualified nanny, maternity nanny, doula, reg childminder, full first aid, food hygiene cert, 15 years exp clean driving licence, 7 seater car and excellent refs! I am also doing my BA(hons) in Early Childhood studies in my spare time. In my ad I offer freelance care to fit in with my own commitment to my family. I have had many responses, so it is easy to pick and choose the family to suit. This summer my daughter will be in Peru on the World Challenge for a month, I am sending my son to France for the same, as their holidys are nearly 9 weeks!
My friend wanted me to go to Arizona with them for 3 weeks at Easter as they didn't want to take the new aupair (too new to share a Winnebago with them for 10 days!) She flies her aupair business class with them all the time as she has three under 7 and DH can't always travel at the same time and joins them later or leaves earlier depending on his work committments, a bit like your situation. - but the children I used to look after are coming to visit me, I am also going to look after a baby I looked after last year until she found a permanent nanny, the nanny has just quit and the mum doesn't want to take on a new one as she is moving north in two months, so I'll fill in.
There are lots of nannies who will do temporary or freelance work between jobs, try the site it is good, but make sure you take up watertight references, written and verbal from past employers etc. Agencie also supply nannies for this type of work but charge a fee, but they do all the donkey work for you. HTH.
As for fees, everyone has their own rate, depends on whats expected in terms of hours, duties etc.