Psychology of children

I've heard children will test limits,will do something you said no to see if you are serious and will stop them. From a young age they will push boundaries what's allowed try to break the rules and it's up to parents to be firm when they say no to something but often children take advantage of parents that are tired exhausted and they try something untill they hurt themselves doing something the parents warned them was dangerous.

Children even with a baby proof house are nuts! they even escape the crib at a year old and two years out of the toddler bed and you take them outside they are covered in mud, inside they cover themselves in food and when they are babies you better change diaper quickly or you could get sprayed with pee or baby poo and be careful baby doesn't put hands in the diaper with #2 or it spread everywhere.

Children are nuts any advice on when you say no and they decide to do it? Ages six and older