I guess I have a slightly different take. Is this a common complaint from your kids? Do they often whine about not being able to have ALL THE THINGS, and get aggro with you for not giving them whatever they want? If so, then yes, some of the harder lifestyle changes and perspective-gathering things might work. And I'd include them in more of the budgeting decisions, as you have started to do. Maybe give them a "clothes and spending allowance" each month, and ALL of their clothes and personal spending needs to come out of that. It's a more long-term goal of helping them see the value of money and the spending decisions that need to be made.
But in real life, take the gifts away or not buying them anything one year isn't actually going to make them suddenly go "gee Mum, you were right! I am now super grateful for the gifts I got last year!" They'll just be reinforced in their view that you are a mean and stingy woman and they NEVER get ANYTHING.
I'd take the long view - this is children we're talking about, not adults whom we can expect to know and behave better, or keep it to themselves if they aren't happy. Especially if they are generally reasonable and understand about money, I'd just chalk it up to actual, real disappointment and envy - not because they really genuinely expected to get everything, but because some other kids are crazy spoiled, and it's hard not to compare.
There are many threads here and in other frugal forums filled with adults saying "but I WANT it, and I'm jealous of my sister and brother and law having it so easy!" or "Help me! I feel really envious and I'm sick of having to slog it out on a low income. It's not faaaaair". These are real emotions, and they're fine.
Your emotions around this are ok, too. YOU'RE also disappointed that you didn't get the holiday/giving experience you were hoping for. But just like your kids are learning, sometimes the event really doesn't live up to the expectation and anticipation, and that's just part of life.
So it is fine for you to be able to say "hey, kids, I know you're disappointed that we didn't buy you everything on the whole list, and electronics as well. In a perfect world, we'd never feel that sort of disappointment, and I'm sorry you're feeling envious of your friends. We know you're both sensible and really do understand that the lists don't work like that, and why we didn't buy the latest gadgets etc. Can you understand that I'm a bit disappointed that the effort I went to to get you gifts that you did want wasn't even given a thank you? I feel hurt that, because you're focusing on all the gifts you didn't get, you've basically ignored the gifts we did give you?"
And just see what happens. You're most likely to get eye rolls or "mumble mumble mumble" rather than gushing gratitude, but parenting is all about the long game.
Good luck. You may not see the fruit of the long game until they're in their 20s. One day of being a sulky kid doesn't mean they're doomed to being narcissistic ingrates for the rest of their lives :)
(P.S I cross posted with Marty998 - +1 to what he said)