How much should I pay??

My grandmother is watching my 2 year old and 5 month old daughter 1-2 days a week..i was wondering what is a reasonable amount to pay her for both kids..she is in NOO way asking to be paid but i insist on it..i know how kids can be...soo witb that being said what do you all think..i just don't want to pay her and then feel bad if it's too little..thank you!