I can't believe I've been a mom for over seven years and I am just now asking about a babyproofing basic, but there it is. DS has recently learned how to open doors, and I really need him not to be able to do that. Has anyone tried those round doorknob-cover thingies? Do they work well? Can older kids (4 and 7) work them? I need to do something to keep DS contained, but i don't want to waste money on something that makes it harder for everyone except him, kwim?