I see no problem with a Retainer Fee.
The Childcare Place is available, the parent is not using that place for a period of time.
What could happen is this:
A contract is signed at the beginning of term for that term, which only allocates that Childcare Place for the duration of that term.
School holidays then come up, and the place can be sold to someone else - who may, or may not need it during the following Term.
To prevent the place being sold, then a retainer fee - or indeed the normal daily fee - could be charged.
NCMA are a membership body, they do not have control over how a childminder operates their business. Childminders do not need to be members of NCMA - insurance, help with paperwork etc is available from other sources.
If a Childminder is using an NCMA contract, then they will need to abide by the terms of that contract. Don't have an NCMA contract to hand, so not sure what the terms are - does it say in the fine print about Term Time Only?