I don't think they'll ever be excited to clean their rooms! lol. I have a 5 year old and 9 year old who share bedrooms. They clean their bedrooms everyday when they get up before they do anything. If not, they get no TV, no video games, no playing outside, no computer, etc. etc. This works for me.
The problem with clean rooms is your problem. I bet your children have no problem with having messy rooms. So I would just be honest and say "Mommy doesnt like you having messy rooms - I would like you to clean them can you do that for me?" If they dont ask them if its ok that you do it or they help you do it.
Let them make their own decisions about cleaning their rooms..because you are the one with the problem not them.
I agree with Mamaada, my boys are 4 and 9 and the first thing they do before coming out of their rooms is make their beds.The have separate rooms and keep them very clean. When they play they always clean up before coming out to do other things or if they choose to play with something different. Example: 4yo plays with lego's then wants to read books,he puts lego's away first and then he can get his books out. It has been this way in our home for a long time and it's second nature to the boys to keep things clean and organized. We DO NOT leave it up to the boys to decide if they want it clean or not...they are the CHILD and have no say-so when it comes to things like that in our home.Having a clean room is not a choice.If you allow them to have a choice in a messy room then what will they learn later in life? That's it's ok to be a slob? I think not, they'll grow up into a person taking pride in being well kept and organized.