When you add a to-do, you'll see checkboxes for each family member at the bottom of your entry. Check the checkbox next to the kids you want to assign to the task. When the to-do is created, they'll be able to view it on their own device and take action from there.
To add members to an existing to-do, click on the to-do to edit it and check the kids' boxes in the entry. You can't edit or change the to-do once action has already been taken, though.