Seriously OP, it is not a big deal at all. They are kids, they are going to do silly things, you need to let them be kids, be creative, build forts, break them etc... The one rule you need to make sure the nanny and the kids know is that they need to clean up after themselves, and make sure they do not break or destroy anything or else there will be consequences. I would suggest you set up a play area or a play room for them filled with their toys and they can do whatever they want there (build forts, jump etc..., but they will have to clean up the play area or room at the end of the day, or else no play room for a day or two). The crayon thing should not be an issue at all, why are we even talking about that?????

Anyways for the laundry, it is not the nanny's duty to do that (unless you are paying her extra), but I would say that on all the issues you stated above, it seems to be the first instance since the new nanny started working, so really I do not see any grounds for firing a perfectly great nanny that lets your kids be kids and seems to take good care of them. Just talk to her and see how it goes. It is really hard to find a great nanny, nothing will guarantee that your next nanny will be better than this one, and you will fire her and the cycle will keep going on because you won't find the perfect nanny. So, just work with the one you have, talk to her, mold her, shape her to fit your family, and if it gets too much for her tell her to be honest and open with you and talk to you and give you her point of view and suggestion on the kids etc. In addition to the parents, it takes a village to raise a kid. Hope it works out for you.