Give your child an even number of easily movable objects (pens, pencils, cutlery). Ask them to half this group by sharing it between you two. See if your child can work out what to do before telling them how to half the group. Tip: the kids loved when we were sharing sweets, maybe try with a small packet of Haribo or chocolate buttons.
Extension: Try giving an odd number, what do we do if there is one left over? (we don't put it in any group because that wouldn't be a fair half, it's just left out)