If I was in your shoes OP, I would do a couple of things.
I would tell my DH that he has to meet me in the middle here, as being a blended family is a work in progress, and he has to compromise too. So, instead of you spending £300 on each DC, spend £225, and he has to spend £225 on his.
What I would also do, is something I had to do with my own DC, and that is count the presents. So, say they get 10 presents each. For the older, that could be a main gift or 2, then fillers. In my house you open a present at a time, and take turns so it is obvious if someone had more. Plus my eldest used to count them!! You can put in a lot of fillers like this.
Your DC are small and they can be distracted with "new, better traditions". I would scrap the Eve boxes and go for one big box. So for me, Christmas Eve is about good food, a big box of chocolates/ sweets, some hot chocolate and marshmallows and a favourite movie snuggled under blankets. Then get the older ones to help the younger ones lay out the mince pies and reindeer food.
If you have them before Christmas, try and do fun things with them. Personally I would try and cut back on the money a bit, and ramp up the traditions and fun so everyone is having a great time.
As an adult I cannot remember any of the presents my parents bought me, but I can remember the things we did together, the places we went and the traditions we had.