When I had an informal arrangement, I charged a premium, used a short term contract with no notice period for either party. If you do activities such as playgroup, parent should really pay their share.
I recently arranged with a friend who is SAHM that if my mum can't do her Thursday afternoon shift for me and I need to work, she will mind my kids (who love playing with her daughter) for £5 an hour, which is just a bit less than I pay the nursery, which is open 10 hours a day and costs me £56 for the two of them for a day. She is very pleased with that especially as its not a permenant 'comittment' as my two boys can be very boisterous.
We are not having a written agreement as then you are in the realms of legal stuff and insurance etc raises its ugly head. I just look on it as compensating her for the invonvenience of havign two extra kids for the afternoon. I don't pay for activities as I just take them to her house and they play there.
GrowlingTiger - it's less than 6 days, so 5 is ok, 6 or more is not.
Munchkinmum - location is all important.
If you care for someone else's child at YOUR HOME, then under English law you are considered to be a Childminder.
If you care for someone else's child, at the CHILD'S HOME, then under English law you are considered to be a Domestic Servant (nanny, babysitter, whatever you want to call it).
So the soluition to avoiding needing to register as a childminder, presuming caring for the child will occur on more than 5 occasions per year, is for you to care for your friends child, at your friends home. You are then a Nanny/Babysitter.
From Schedule 9A of the Care Standards Act 2000
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3.(1) Where day care is provided on particular premises on less than six days in any year, that provision shall be disregarded for the purposes of Part XA if the person making it has notified the registration authority in writing before the first occasion on which the premises concerned are so used in that year.
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This does say Day Care rather than Childminding, but I would consider it the same thing in this regard - unless anyone can find any document which states a different number of days per year for exemption when the day care is provided on domestic premises (childminding).
For those interested in more information about what is and isn't a childminder, see HMI 2601a - page 7 if viewing the PDF.
Should add... this does assume that the child concerned is under the age of 8 years old, and that you live in England.
I would add that mon tue wed I pay full fees at a nursery. I am self employed and mostly work 3 days a week - sometimes work Thursday or Friday too, and my mum looks after the boys. I just wanted to be covered in case of my mum or dad being ill, or on holiday, then I don't have to turn work down or let clients down if I've already agreed the work. So I don't think it would be more than 5 or 6 times a year.