I can feel myself getting
about this already.
Every time the DCs have a birthday or its Christmas my dad always makes comments like "wow you've spent a fortune" - "that must have set you back a bit" as they open their presents. I expect more of the same this year as I am now a LP and so doing the whole thing on a tight budget. Actually, the majority (and sometimes all) of their presents come from jumble/NCT/carboot sales or my charity shop trawls through the year. Quite a few of their "presents" are things like tights and pjs which they need anyway. I know it is because he grew up in the depression with a fiercely stingy mother but it is starting to piss me off mightily. I know my kids have less than any of their friends (not judging those parents - we have what we have) and that he has lost any sense of perspective. In years past, I have just said "you know I buy 2nd hand mostly don't you" but he ignores me. He and XFIL (now deceased) were a real double act on this 
What I don't want is for him to spoil it for my kids. They do not have hundreds of presents. Their main presents cost £15 & £20 new (no competitive underspending here - they got what they asked for) plus assorted 2nd hand goodies and stocking fillers totalling about £10
I suppose I object to his comments because firstly, I have not spent "a fortune" and second because I think the DCs are of an age where they will pick up the vibes (3 and 5).
So WWYD? Raise it in advance - quick phone call or visit or just bite my tongue and smile on the day? The other option is to write him a note which sounds awful and a bit passive aggressive but he is somewhat deaf and "tunes out" a bit sometimes even when he is wearing his hearing aids 