Depending on the children's ages, if they have matured a bit, have had some exposure to life, and are possibly able to operate and use the internet, you can teach them a lot about Bitcoin so that they can handle the private key on their own. After teaching them, you might as well create separate wallets for each of them so that you can send each of them their respective shares so that they both have access to them. But its not wise to teach them before they get of age so it won’t be a distraction to them and their studies.