You made the decision to let your children carry on living in a world without that terrible tragedy in it.
You let them have a holiday and then you told them something that children shouldn't have to hear or have to come to terms with. That's big. That's not evil of you. That's being a parent, making decisions for the good of your children. That's not done selfish or inexplicable lie that would come back to haunt you and them.
Maybe you selfishly wanted to have that nice time with your little ones, that you couldn't bear for it to spoil a much longer for or saved for holiday? Maybe that's why you feel such huge guilt? But that's ok to have felt like that if you did, you are a human, and that would be a very human thing to do.
I know something about not being able to make your child's life what it should be, and having to be the one to change their world by telling them something really bad. It's something I struggle with everyday, not for the same reasons but with a similiar result.
All you can do is help them understand at the right moment and in the right way, and just be there to help them deal with the emotions you can't save them from. And that's what you did. And what I'm trying to do.
If my ds grows up to tell me I did anything right like that, I'll know I did ok in my flawed and human way. I think you should grab onto that, what your children think.
I didn't tell ds my father, his grandpa had died. Not for days. I didn't want him to feel my grief, my pain and I just wanted him to live in a world where he had his grandpa, who was more like a father to ds than a grandpa, just a bit longer.