To be fair, £100 per child doesn't buy much these days, especially once children are a bit older. I spend roughly that and still only stick to the supposedly scroogey rule of one thing to wear, read, want, need, and I don't buy designer gear.
One the books alone was £30, though (new release, so only available in hardback), and once a child reaches adult clothing size at around age 15, suddenly a top is £25, and let's not talk about a pair of halfway decent trainers at £50 at the cheaper end.
Even at £100 per child I still, occasionally, feel mean when I buy them a set of pyjamas to wear because they've grown out of their old one and wrap it up as a present. I'm not on a low income, but I am a single parent, so all my income gets swallowed up by bills I have to cover fully by myself and therefore even buying new clothes is a bit of a special occasion.
So I can fully see what the OP is saying with regards to costs. Yes, there are many ways of having it cheaper. Nothing is a must at Christmas, not even a Christmas meal, but children will get asked what they got and how their Christmas was when they socialise with their peers.
Where I disagree is the concept of a bonus. We need to see benefits for what they are: a safety net that enables survival. As such, they need to be enough that people can pay their bills, but no more. Bonuses come from employers as a "thank you" for hard work, or because the company has done especially well. That some workplaces rely on UC to top up their own poor wages, which don't even enable survival, is a different matter that needs to be addressed with them by the government.