Tax credits on maternity leave

[Pearls18]

Do I have to inform TC I am on maternity leave? I am assuming I should obviously tell them asap when our childcare costs reduce due to me pulling LO out of childcare for most of the week. If I am still classed as working will they still pay some if I decide to let him to a couple days? (he will get his 15 free hours but I will probably have to pay for the outside hours and school holidays etc to reserve his place as it is a private nursery)