It's actually great news that some of it is in a discretionary trust and some in a bare trust. It means that your son can't squander the whole thing.
But honestly, OP? It sounds like your kids have a generally healthy attitude towards money, and you should give yourself a bit of credit for that. They don't go without, but they also don't sound spoiled or entitled.
The next step is making sure they're financially literate adults. One of the things I'm most grateful for was a series of conversations I had with my parents when I was about your eldest's age, which were about preparing me to understand basic financial principles. So, for instance, I learned that the most important decade for pension savings was my 20s, because it had the most time to compound (in fact, my parents topped up my pension when I was working my first, very underpaid, nonprofit job, something which I'm still grateful for). I learned the basic ideas about investments, risk, property, estate management, trusts, taxes, etc.
It was done in such a way that I wasn't overwhelmed. All very much "this isn't something for you to worry about now, but it's something you should know about for when you are Grown-Up-in-the-Future-Many-Years-From-Now."
They also told me they'd manage everything for me until I had my feet under me a bit more, which I was so grateful for that it never occurred to me that I could take a chunk of money and blow it.
I absolutely did dumb financial things in my late teens/early 20s, but they were harmless: I bought expensive books when I could have taken them out of the library; I treated my friends to birthday dinners at nice restaurants (and was treated in return); I bought some earrings I fancied even though they were dear. But these were £50 or £100 or maybe £200 mistakes, not £10,000 or £100,000 mistakes. Much more slightly indulgent than Blair Waldorf.
My parents' help wasn't in direct cash but in taking care of things for me. So, for instance, after I graduated university they covered half my rent in the city (I was at that time living in a tiny studio in a safe but slightly gritty neighbourhood, so this was really a nominal amount but meant a lot to me), and as I said, they topped up my pension to maximum nontaxable amounts. They also helped me organize my taxes/the trusts. But I paid my own bills from my own chequebook, my own groceries, my own credit card. I felt like an independent financial actor, and I never for a day expected to be rescued by any inheritances (they all seemed quite a long way off to me).
I will say, in contrast, that my second cousins were spoiled beyond belief by an expectation that they'd never have to work a day in their lives. I'm talking crashing sports cars and immediately getting replacements from their grandparents; not going to university; funding dodgy friends' investment schemes, and each time getting bailed out with family money.
On the other hand, even though I never went without and lived a very generous life, it was always clear that my parents expected me to use my energies to a purpose. We were expected to be ambitious and to be kind. While I was slightly jealous/incredulous at 16 about my cousins' seemingly endless indulgence, I am now mostly really sad about their rootless, aimless lives.
tl;dr: see financial literacy as part of your kids' overall education; keep up the same standards/expectations you always have; make it clear that this money is for later; and do what you can to manage things for them as they figure out who they are.