The big mistake a lot of people make is to put the child benefit and other money into a deposit account on the basis that they are "risk averse". The risk might be quite small over a year or two, but the longer the period the greater the risk that your savings will lose a lot of their real value due to inflation.
In particular, it's crazy to have money on deposit at 0% while paying 3% interest on a mortgage.
If you have €10,000 today in a deposit account, it will be worth €10,000 after 10 years, if it earns no interest.
If you pay that off your mortgage today, you will save €3,400 compound interest on it for 10 years.
In other words, your mortgage will be €13,400 lower than it would otherwise have been.
(You could invest the €10,000 in a fund, and this would be better than leaving it in a deposit account, but you might not get a return of 3% tax-free on it. The return could be higher or lower than 3%, and it will be subject to charges and taxes. So paying off your mortgage is better than investing in a fund. )
If your mortgage is lower, then your repayments will be lower so you will have more money with which to pay college fees.
Paying the money off your mortgage has the added advantage that it will reduce your Loan to Value ratio which means you could get a lower interest rate on your entire mortgage. Or if you trade up in the meantime, having a lower mortgage will help you trade up. You might trade up just before you need money for college, and could at that stage take out a bigger mortgage than you need so that you can use the extra cash for fees.