Boundaries are best taught as body awareness, not rules. Children do well when they learn to notice what feels comfortable or uncomfortable and trust those signals.
You might say, “Your body gives you information. If something doesn’t feel okay, even if you can’t explain why, that matters.” Reinforce that boundaries apply to everyone, including adults and family members. Let your child know they are allowed to say no to hugs, tickling, or physical affection, even from people they love.