Can you pay into another adult's savings account?

[busymyfoot]

My inlaws want to start gifting some money to my children, age 19 and 17, on an adhoc basis. I don't know how much or how often, but they want it to be saved towards a future house deposit. I know they could gift into my 17yo's Junior ISA, but only until he turns 18. My understanding is that after that they would have to gift it to their current accounts and rely on them moving it somewhere sensible. Is that right? Or are there adult savings accounts (with decent rates) that can be gifted to directly?

Ideally I want to find accounts that don't have restrictions on how much can be paid in, or how often.

(To avoid drip-feeding, DC19 already has a LISA and an adult ISA, which I contribute to occasionally by transferring to his current account and then helping/encouraging him to move it, so I'd prefer not to have to do the same for the grandparents' contributions, if possible).

You can pay into any account that has an account number and sort code, as long as there isn't a limit on the total amount in the account. But the account would have to be set up by the account holder not a third party.

But the inlaws shouldn't be gifting any money to anyone if they want to put restrictions on how it's going to be used but don't trust the recipient to abide by them.

I'm finding that the adult isas and savings accounts I am looking at have restrictions on who can pay into them - probably it's an anti-money laundering thing. They all seem to want payments from a linked bank account. Exceptions seem to be LISA's and JISA's, which allow gifting if you fill in a form, but those aren't good options for us.

If anyone has first hand experience of a provider that definitely does enable gifting into adult savings accounts and isas, please let me know and I'll take a look.