This is a recurring argument every year. DH buys his father a gift from him, I buy my father a gift from me and the DCs choose something for DH.
A few years ago BIL took up with a woman who already had a DD and since then they have also bought FIL a gift for “grandad” on Father’s Day (same for MIL on mother’s day). ILs are very welcoming and do treat this girl the same as our DCs which is nice.
Anyway every year since it’s being brought up how awful it is that we don’t buy FIL a gift from our kids. I usually just do my best bewildered face and say “but he’s not their dad...?” This year MIL has text outlining her upset and that we are thoughtless etc if “even Molly knows to get him a gift”.
AIBU to think Father’s Day is for fathers? We have a good relationship with FIL and MIL and make sure they know they are appreciated throughout the year. I don’t want to cause issues every year but I don’t see why I should buy gifts I don’t want to but either 
*Molly is not the child’s real name.