Depends on the insurer. The child benefit is normally 50% of your serious illness cover at the time of claim subject to a max child serious illness benefit. For example if your policy states the max child serious illness benefit is €25,000 and you have €100,000 cover on yourself then 50% of €100,000 is €50,000 but the max benefit on a child is limited to €25,000 so €25,000 is the amount that would be paid out. Policies differ so you'd have to check your policy terms and conditions.
Remember when you are putting in a claim always continue to pay your premiums.