ferry23
I suppose what you do next hinges on the type of family dynamics you have, but if you want to gift money to your grandchildren as a kind of nest egg for when they get older, isn't that up to you how you choose to do it?
I'm not sure why you feel you have to keep consulting your son about how you should build up the savings for them. It's your money to start with and your grandchildren are the recipients, but it seems your son is in control of it. Whilst it's always wise to take a bit of a step back as a grandparent and accept that you're not the parent, it seems a bit extreme to me that you can't choose how to treat your own grandchildren.
But whatever you do next I'm sure it will all smooth over.
As I understand it, you can't open a bank account in someone else's name unless you have parental responsibility. So, you can do as my mum did, and open an account in her name, specifying in her will that 'account no. xxxx is for Joe Bloggs'; or you can ask the parents what they are doing about bank accounts and get the details for paying in.
It seems to me that there was a muddle about the purpose of the money in OP's case. We have always specified ' this money' (a fairly small amount at birthdays & Xmas) is for you to spend however you want' and 'this money' (a regular payment into an account) is for the future.