Kids love to stick things on their walls. Even though we hate to fill in all the little holes, it gives them so much pleasure why not give them some designer push pins to tack up their posters and what not.Even better, buy them or make them their own place to keep mementos and pictures by using cork squares. You can double them together to make it thinker, cover it with a pretty piece of fabric and glue it on the backside. You can use a couple of strips of ribbon going over the fabric to give them another way of holding pictures there. They just slip them under the ribbon.
You can put a wood frame around the cork tiles if you wish since it will be easier to hang on the wall. Then buy them some kewl push pins to stick things on their new display.