Depend on contract. If you have 10 day personal paid in your contract, you could use those for your mat leave. If you not have paid leave in your contract, then that not part of your agreement and you cannot charge parent. What does your contact have for time off?

EDIT - Just reread and see you don't charge when you are off. In that case, this same situation. You are taking time off, it not matter if that for vacation, sick, or mat leave. Your contract is you not available, they not pay. You not able to charge them. That not your contacted agreement.