My mother in law is flipping out bec we let our 18 mos old mark on walls with erasable crayon. Who is wrong?

This is all a questions about over-permissiveness. I dont' think its that big of a deal. The kid enjoys it and we have a girl who comes in and cleans. The stuff comes right off. Mother in law thinks we ought to force her, an 18 mos old girl, to clean it off herself. I think the old lady should mind her own business. What do you think?