Work an extra hour

I've started a job working with kids the whole day. I'm typically very exhausted by the end of the day usually due to having an early start. I've been asked to work an extra hour and for some reason panicked. I love work however the idea of simply staying even an extra hour past what the shift originally was causes me great stress. I don't know how to help myself in this sort of situation, maybe it's because I'm weak but working for even half the day is tiring for me having an extra hour may seem like nothing but I don't seem to be able to take it like that.