Re-read OP's posts.
She supplemented the things in the 'xmas eve box,' with family money. So GPs bought 2x hot chocolate and 2 x pyjamas and OP bought another 2 of everything for SC. Fine, lovely thing to do, and yes for the sake of fairly cheap items it would have been nice for the GP to have just done it for everyone, and petty for them to begrudge OP doing it.
BUT that is completely different to GP giving OP £100 to spend on Christmas presents/an outing for their DGC and OP only spending £50 of it on them and the rest on the SC and not telling them about it, which is what OP has been doing.
For the last - what 6 years or so, that's £300 OP has pocketed and spent it on something else. Yes, the "something else" is a lovely thing to do for the DSC, and at 6/4 her DC won't know the difference between a £25 day out each and a £50 one, but in 5 years they will if they ask for a £50 present and only get a £25 one!
It's exactly the same as the GP giving OP £100 to spend on the kids for Christmas, and OP only bought them presents worth £50 and spent the rest on a present for herself, or Christmas dinner, or a few bottles of wine while telling the GP she'd spent it all on the kids.
While it would be lovely if the GP did spend the same on all DC, they aren't obligated to. They don't feel like the DC are part of "their" family, and nobody can make them feel something they don't. If OP feels that strongly about it she should refuse to take any money at all, but you absolutely can't take money off someone for one thing, lie that you spent it on what they gave it for, but actually spend it on something completely different. That's completely immoral, even if you think you're doing it for the 'right' reasons.
If you donated £100 to cancer research but found out £50 had been given to the donkey sanctuary, or Mermaids, or Salvation Army, or whatever instead, would you be happy with that?
It would be different if OP had done the same as with the stuff in the CE box - spent £50 each on her DC and then paid out of family money for the other DC to accompany them, but she's admitted that's not what she did - she split the £100 between the 4 DC (in fact if anything often older children cost more for things like the examples she's given, e.g. light displays). The grandparents probably wouldn't care if the SC experienced the same thing as their DC, they just don't want to be the ones paying for it, which, even if unpleasant, is a choice they are entitled to make.
I don't blame them for being pissed off that they have effectively been stolen from and lied to.
Also, while the DC love their siblings and might prefer the money is spent on them accompanying them now, it will likely be a different scenario when they are 21 and they receive £100k from their grandparents in their will - as much as they love their half siblings they probably won't be offering to split that with them, so at some point down the road OP is going to have to address this and not keep pretending that her parents love the SC the same.