Good morning!

This is not a huge problem, but just one that gets on my last nerve on a daily basis.
I have a little DCG who I adore! I knew I would love her the second I met her. Her parents are very nice people and I don't have any problems with them except for the fact that they continue to wear very nice clothes on her to daycare. I have in my contract that I am not responsible for any damages done to clothes. I ask parents to wear clothes which are not 'cute' or 'nice' as we do a lot of painting, markers, and outdoor play. This particular child does not walk yet, so when we go to the park, she sits and crawls in the sand/grass. Today for instance, she shows up in a beautiful white long sleeve shirt and white jeans!!!! I've asked them on numerous occasions to please not send her in nice clothes...no such luck. I am planning on taking them to the park in an hour...should I just let her sit in the sand and let her jeans get dirty? They do send other clothes which are nice but of a darker color but I don't think I should have to change her everyday before we go to the park because they insist on wearing 'pretty AND expensive' clothes on her. What would you do?? At this point I really want to let her get them very dirty to send the msg. Is this really unprofessional of me?