A few years ago there was a high profile case where 2 police officers got into trouble for a reciprocal childcare arrangement. However since then, the law has changed. In 2020, there are two main ways that reciprocal arrangements can be made without requiring Ofsted registration. When using the Family Finder, please be careful to stay within these situations.
You can look after your child or children along with a child or children from another family at the other family's house. There are no time restrictions on this, and it can be either paid or unpaid.
You can look after the child or children of friends at your house, as long as you don't get paid. There are no time restrictions, and the children involved can be of any age
You can do both of the above things on a 'reciprocal' basis, e.g. One parent takes care of all the kids on Monday and another parent does Tuesday. This reciprocity doesn't count as 'payment'.
You do need to be careful to avoid 2 situations which are not allowed:
Being paid to take care of a child that isn't a family member, at your own home, if the child is under 8 years old and the care is for more than 2 hours per day. This requires Ofsted registration as a childminder
Looking after the children of more than 2 families at once. This requires Ofsted registration as a childminder.
This answer was checked with Ofsted on 11 March 2020. For a useful table with more details on registration, see more detail here
https://www.oxford.anglican.org/wp-content/uploads/2013/02/Registration-not-required1.pdf