Buying useless things for small kids

My daughter turns 5 soon and her grandmother and her sister wants to get her a bike, they are insisting. Let me point out that we have 3 different sized bikes for her already. 1 she got brand new the other two used from friends. They are just sitting there!!! She’s not too interested in them anyway and the last thing we need is another one. I’m really annoyed at this, at least spend money on what she really would love or invest that money in her funds. How can I politely tell them no??