I dont want to sound ungreatful but I cant help but feel a little peeved by this.
My inlaws mentioned back in june that they had seen a bike in a 2nd hand shop that they wanted to get DS for Christmas, I politely tried to put them off as DH and i had just bought him a new one in April for his birthday. I found out yesterday that they went ahead and bought it anyway. Im annoyed as its too big, DS is 3 and the bike is easily more suited to a 6/7year old. Im annoyed because it seems like such a waste as he has barely even ridden the one we got him, so i just know that this one is going to be a wasted present.
There is no problem unless they won't store it. It's useful in a few years time. Save you buying one then. Money saved in future providing you don't have to have it at your house. Stand firm on that one.
My ILs buy large gifts for 3yo ds that we cant store or repeat gifts that we have at home. It just gets left at theirs. Ds doesn't even remember generally. Tell your ds he has lots of things to play with a home, including a bike. Say this will be a bike for gp's house.
If by some fluke he remembers next time you go then get the gps holding him balancing him on it. It might hurt their backs! (Sorry, mean). My ds loves balancing on my neighbours boys' bike even though he can't pedal it & he has a balance bike he fits perfectly.
If you encourage your son to be adaptable he will be. Otherwise you're just setting him up to have tantrums over nothing.
When I have grandchildren and buy them presents I would like them to know I have brought them the present. My parents buy my DC a selection box of sweets every year as their present so you do sound a bit unreasonable to me. However I would ask the grandparents to store the bike for a few years rather than hVe it cluttering up the garage or shed.