I take my kids to the laundromat with me, too, and they do the same thing with the carts that they have, and the running up and down...
Think about the environment... Obviously those carts weren't put there for childrens entertainment, but does a child understand that?? It's a big thing on wheels that you can hold things in!!! Let's have some FUN!!! And there is so much open space for them to run!!!
AND IT'S A LAUNDROMAT!!! As long as the children aren't pulling other peoples clean clothes off the folding tables or out of the washers and dryers, then there is no harm being done to them.
I would do what you would do if I saw another child "misbehaving" (though it's not misbehavior, in my opinion, if they are just acting their age!!) Show that I care for the childs best interest (after all, it takes a village!) and if the mom doesn't respond, I'd make sure that *I* was watching that child... maybe distract him/her from whatever "wrong" behavior and try to get him/her to join in with something I'm doing, or that my children are doing...
NOBODY has a right to tell you that you aren't taking good care of your CHILDREN!! And the laundromat is a pretty safe place for your children to just do whatever with you not totally focusing your energy on them...
(my laundromat people don't like it when my kids play with the carts, either... I don't understand it, but I make sure that when they comment on it, that's when I ask my kids to stop... it's harmless fun, though!!!)
(I totally burnt my toast writing this!!! ARG!!!