Well it looks less clear cut in view of what is (gradually!) emerging. But - while I don't doubt your motives - there seem still to be a few possible issues.
An "allowance" paid under deduction of tax which they can then claim back from HMRC? Interesting. I assume neither is studying for accountancy or CIOT qualifications :)
OTOH you also leave them in the dark about their prospects which means they make less than optimal life choices. I do hope for your as well as their sakes that you don't face in due course "if only you had told us....." The more so as you have not indicated if the trust deed makes it impossible for capital to be advanced before they reach age 25.
Covert indeed given they are now over 18. Trust law requires a beneficiary to be able to enforce the trust. A beneficiary can hardly do that if he is unaware of the trust.
But it may well all end well - and I do hope it does.
reply to address is (meant to be) valid