I think part of the issue on this thread is that actually, setting up 'child savings accounts that grandparents can pay into, without disclosing the quantity' isn't always quite as straightforward as some people seem to think it is.
I set up an aptly titled 'Kids saver' with a high street bank when my son was a couple of years old, which we have direct debited a small amount into monthly ever since. It pays a fairly decent interest rate, for total deposits capped above the amount we were able to save.
When my son was 7 my MIL stated she wanted to start some regular savings for him, and did we have a bank account for him she could send it to? She wouldn't tell us how much money she wanted to save, so in the event that she wanted to save more than the capped amount per month for his 'Kids saver' account, I set up both a cash ISA and a stocks and shares ISA in our son's name.
I then went to sort out the account details for her....
The Kids Saver one, despite being for under 16s and requiring a lot of my son's details to initially set up, is actually in my name. I queried this twice with the bank and they say this is how they set them up.
Its also not the case that my husband and I can't access this money. In fact I withdrew a lump sum to set up his ISAs with last year, via our joint account, as it was the simplest way (and from what I could work out the only way) to do it.
It is, once in the ISAs, 'his money' and is unwithdrawable until he is 18. But that wasn't the case when it was sat in a normal, high street Kids Saver account.
For both the ISAs I set up, one an NS&I children's cash ISA and the other an stocks and shares ISA, as neither are current accounts, you can't carry out simple BACs transfers to them. They involve a fairly complex number of steps for people who aren't the 'registered contact' for a child's ISA. And only a parent or guardian can set up an ISA for a child and be the 'registered contact'.
Its actually quite hard to pre-grease the wheels for a grandparent to deposit unspecified sums of money, with ease, for their grandchildren in a way that keeps that money ring-fenced from the dastardly DIL who bothered to set the whole thing up in the first place...
OP, maybe your husband needs to start having a perusal of the Money Saving Expert website and sort it out with his Mum?