france
In France, not only is it legal for parents to take items from their (minor) children, it is also legal for children to take items from their parents or for parents to take items from their adult children (article 311-12 of the Code pénal). The same is true between spouses. Basically, the law treats this as a private matter families have to sort out themselves.
You did however mention a cell phone and the law was recently amended to make an exception for that. It is illegal to deprive your children, your parents, or your spouse of the “means of communication” they need in their daily life. It's not obvious whether this also applies to young children.