I pay £20.00 pm into each of my 3DSS savings accounts, and my DP does the same. They also get a cash lump sum from my parents each Christmas, (they do this with my nieces, and give the DSS some too, but this is less). Which I also put into their savings account. I set it all up, and my DP was happy.
We haven’t told the DSS mum though. My DP use to pay into a savings account they had since birth. However we have now learnt that this money, plus other money for DSS has been withdrawn and spent by their M.
They use to also receive money in cards for Christmas / birthdays from DPs family, this money was never received by the DSS and never put into their bank. All DPs family now give us the money directly and we put this into the accounts we hold.
The eldest DSS even brought his birthday money (£80) which he had received from M’s family, and asked us to put it in his bank. (He is aware of it, as we discuss the importance of savings, and I’ve taken them to the bank to put money in before). His M went mad, saying it was “her” money to deal with. We ignore it and followed the DPs wishes, I think he is just wise that it gets spent.
Don’t worry about doing something for their future. Many SMs do, and it’s a lovely gesture, no matter of the final sum.