Maternity Allowance question

Hi

I'm really hoping that someone here will be able to help me as I'm very confused about something.

I currently look after a brother and sister who's mum is a teacher, so I care for them term time only. However as mum was going to struggle with the cost of childcare I agreed to spread the cost. So I have spread the cost of term-time care over 12 equal payments meaning that mum has lower monthly payments and I get a payment each month (regardless of school holidays etc).

My problem is that I'm currently pregnant and plan to start my maternity leave when the summer term finishes (so have the 6 weeks holidays off) as I'm due first week in Sept. I obviously want my mat allowance to start straight away but i'm concerned that because I will be receiving a payment from this particular parent over this period they will say that I'm not entitled. But my argument is that she is paying me for work that I've already done and I would also usually take on another mindee as holiday cover so I will be losing earnings.

Do think receiving this 'back payment' will mean that they wont pay me any mat allowance?

I hope I havent waffled too much and that it all makes sense.
Thanks x