My apologies for the delay.
I have been hunting for a suitable template for you but most of them are American where such things have more legal force.
A Foster Care arrangement is where a child is placed with another family to be looked after instead of remaining with the biological parents
In the Uk this is usually arranged by Social Services.
However it is possible for there to be private fostering
You can read more here
However as you have realised the Foster Carer will need authority to deal with Medical and Educational issues
You can adapt the template here
To suit your situation.
In the UK there is no need for it to be notarised
I hope that this is of assistance - please ask if you need further details