Help, can I have a normal Yale locking door in my low rise flat?

Hi, I live on the top floor of my low rise flat, it only has two floors! My door Is a fire door that opens from the outside with a normal Yale lock key but once your inside you can't use the key, you have a little nob that you turn to lock the door (obviously so if there was ever a fire and you couldn't find your key you could still open the door), my son is four and he knows how to open these doors and last week my neighbour found him on our communal landing late at night, he has wondered out while we was sleeping!! As you can imagine this is really worrying for us!! Can I have a normal door in a flat that you lock on the inside and out or is it a legal requirement to have these doors in? Thank you