There is a lot here that I would like to question - not trying to be unkind but it just feels like you have a bee in your bonnet about this and are not thinking clearly:
A) I would be amazed if the 12 year old, your dp's youngest, has memorised the prices of 5 year old kid's toys, to the extent that she could sit and instantly calculate how much you had spent on your kids compared to her. And if she is capable of so doing, then she is VERY much capable of understanding and factoring in the cost of the presents she gets from her mom.
If she wilfully insists on causing a massive problem about this, despite clearly being able to understand there is no unfairness here, then you have a much bigger problem than Xmas morning. She would have to be a deeply unhappy child, to behave so irrationally.
B) you genuinely think it "ruins the magic of Christmas' if children only get £150 worth of gifts in the morning, and have to wait a few hours for the other £150 worth? I mean, seriously. Does Christmas have no magic, then, for the massive tranche of the UK's population of kids who don't get £150 worth of presents AT ALL, and would cheerfully wait for them if such a bounty were on offer.
If this would destroy "the magic of Christmas" for your family, I suggest you refocus things, because this sounds deeply worrying.
C) despite being apparently so concerned about this issue, you admit you "can't be bothered with the faff' of putting a few presents in a car and taking them somewhere? What possible difficulty or trouble would thus cause you?
Sorry, I know this probably sounds a bit aggressive and I don't mean it to. I just feel like a lot of us have spent our time and emotional energy to answer your query with advice in good faith, yet a lot of your replies sound like you don't actually want to find a solution. Forgive me if I am wrong.