I would have thought £150 per child sounded totally normal. Hell, every man and his dog appears to have a Christmas Eve box as well and they're £20 or so.
They are joint presents - so I would have to consider what I would spend on my own children and buy together. If I lived with a man and we had joint finances but he would only contribute £20 per child and expected me to contribute the rest when I wasn't earning I would be unimpressed. (Assuming he wasn't totally broke which it sounds like you're not)
In your shoes I would spend less on your SS than your SS and I would spend less on both of them if they lived full time with their mother. If they live full time with you it's your joint responsibility to give them a good Christmas.
You don't have to spend a lot - if you can buy second hand, bargain hunt, get thoughtful presents where time is involved as well as like a promise of a day trip to X or materials towards Z project.
Considering the games consoles, laptops and mobile phones some children get I don't know why £150 is considered extortionate.
I would roughly break it down to £10 - 'Christmas' spends (mug, pjs, DVD or book), £5 chocolate, £15 stocking fillers (for older children this can be more expensive), One big present like headphones - £40, branded clothing x2 , £50 - that's £120. Plus a couple of other bits to open like an iTunes gift card - they all add up. If she's not having Christmas day with you or you are not the main home giving her presents obviously these costs can come down.
That's when you are more frugal through the year, when I was a child Christmas presents were always more because I got hardly anything through the year!! If you're always supplementing your SD shopping and going out etc then you can take it easier and just get her a couple of fun items.
This will be a lot for some and too little for others but I don't think it's extravagant.