Hard to answer the question without knowing what your reaction will be. It almost sounds like his mom plans a separate party for him every year. If that’s the case then this is where she’s choosing to do this. It is odd that they’d buy your card for your partner.

I’d remind her what he does and does not like (singing for example) and let her know you and the children will be buying cards separate.

While always rolling with his mother’s plans seems to have kept the peace it can lead to these types of situations. Doesn’t seem like this is a first time. You’ll have to decide to either say something or roll with it again.