Employment status - creche staff

Here is the scenario.

A pool of workers are available to a creche, shifts are offered, and accepted by pool members, but there is no obligation to accept hours in order to stay on the list. There is a contract, but it specifies that no hours will be guaranteed.

The workers all earn below the NI limits (max £4k per worker per year)

When they are working, they are generally in sole charge of the operation.

Would you consider these people employed or self employed?

The business has previously been advised self employed is appropriate,

Thanks in advance,