The OP said that her Ex H pension contributions take him out of the 40% tax bracket, which is roughly 50-125K . And it’s two sets of school fees, so let’s assume average fees of 18k, so 36k. To make the maths easy, lets say he earns 80K and he's putting 40K info his pension, which puts him below the 40% tax.
To calculate if this is excessive , the CMS look at the age he started paying into his pension, let’s say that that was 35. So it’s reasonable to pay 16-22% of his salary into his pension. But he’s actually paying 50%.
Then they look at his projected private pension, so you need his age now, his age at retirement, the amount he has in his pension pot now and whether or not he will get the full state pension. That allows you to calculate how much his projected pension will be.
Then you look at the benchmark figures for what % of hits current income he will need in retirement. For 40K thats 60% . So he needs 24K a year to live at the standard he is now.
If the projected pension income is a lot more than 24k per years ( you take account of the state pension of about 11k if he will get it) then you have an arguement that he's making excessive pension contributions.
Yes it’s a good thing to save into your pension. But not at the expenses of supporting your children now.