This is bizarre.
You plan to deposit €20k at 0% so that you can borrow €20k at 2%? You won't have access to your savings as the credit union will not allow you to withdraw it.
Buy the car for cash. And with the €300k repayments build up your savings again.
In case of emergencies, you will have the kids' fund to access.
Do you have a mortgage? If so, and it's not on a tracker rate, you should pay the kids' fund against that. When the mortgage is paid off early, you will have more money for your kids' education that you would have at close to zero deposit rate.
Brendan