You have an interest in music and love seeing your grandchild share that hobby.

If my parents did that for one of my nephews/nieces I would be happy for them, and not be surprised. It's unrealistic for another grandchild with a different random hobby to expect the same type of financial help; rather it sets a precedent that you are willing to help for that specific hobby.

Anyone who complains needs a kick in the pants. It's not undercutting a provider's role, and it should not create jealousy. If it does, it's poor form on their part.