Hello, I'm Keith and happy to help you with your question.
Child tax credits are only paid, according to the Government's web site, under the following conditions:
'The child’s age To qualify the child must be: Under 16 - you can claim up until the 31st August after their 16th birthday Under 20 - if they’re in approved education or training'
Thus you would be entitled to the benefit for the younger child, but not the sibling unless in approved education or training. Furthermore child credits are enhanced if:
'You may get extra Child Tax Credits if: The child gets Disability Living Allowance or Personal Independence Payment.'
To compute whether you would receive any child benefit you must use the tax credit calculator available on the Government web site. There is such a plethora of information needed to undertake this that it is simply beyond the scope of this question. Please look at Child Tax Credit on the UK GOV web site. However, at your income level I suspect that you will not qualify, but cannot be sure with this excessively complicated benefit.
I am so sorry not to be able to provide a definitive answer, only able to point you in the right direction. Furthermore if the Tax Credit Office have made an error in their assessments and over paid you they are entitled to ask for its return; more gloomy news, I regret.