I'll work it out for you based on what's "normal", ok?
£100 per child for birthday gifts
(As they grow older they prefer more cash and fewer presents so your 9yo would probably get £80 gifts and £20 cash while your 16yo would get £80. Ash and £20 gifts)
£100 per child for Christmas
You buy their school uniforms, genus don't need to know anything about it other than that they have uniform
You buy their basic wardrobe ie underwear, everyday shoes, trainers, summer shoes, winter boots, jeans, shorts. , yes, hoodies, dresses as and when needed
They needn't be reminded of the cost unless they are asking for premium brand clothes in which case they may have an allowance of say £5-£10 a week each according to age. (General rule of thumb is $1 or 50p per year, per week so £6 a week for 12yo, £8 a week for 16yo)
If they wish they can use this allowance to top up clothing spending and therefore buy premium brands.
For example, I'd give my 15yo daughter $15 a week which she could use to pay the difference between $70 trainers and $110 trainers. Equally she could use money from her Saturday job. But so long as she was at school, I supplied all her essentials.
Where I am school uniform is very expensive, $1000 a year, and there is no reason why my children need to be worried about that. It's an adult concern.
I'm not sure how you have managed to complicate this to such an extent. Just pay for the things they need and give them an allowance for the things they want. Birthdays and Christmas separate.