I'm nana to my Step daughters little boy. I love him to pieces. I have offered to help and have him sometimes if she wants a break or has things to do etc.
She will always message or call my husband though (her Father) and ask if he can take a day off work to care for him (if she has an appointment or needs the odd days childcare etc). I work part-time 2 days per week and so I'm much more available to help out. It just seems strange that she doesn't ask me and goes straight to her Dad as it would require him using annual leave whereas I'm pretty flexible and more available.
She doesn't have a good relationship with her Mother unfortunately. We have always got along - we're not particularly close, but I thought she would feel more than comfortable asking if I can help with childcare, as I've always been involved and offered. DH will take a days hol, but then I'm off anyway, so seems odd to me, like I'm looking after him but only because DH is present!