Sorry, but......even though I'm in for a flaming here, I know, for daring to say something not entirely gentle and supportive, I cannot agree with you, Patty. After having reread your description of the events, my conclusion is that your children were being disruptive to others and were acting inappropriately for the setting. This is based not only on the rude couple's remarks, but also on the remark of the employee who was telling your son to stop playing with the cart.
My sympathies go out to you because having one child, I can only imagine that it must be at least twice as hard to have two. I think the couple approached you in a manner calculated to make you angry and defensive, and that their approach certainly could have been much more effective had they been polite. However, the obvious fact is this: a laundromat is not a playground; carts are for laundry, not playing in (however "fun" they may be or appear); and worst of all -- and most important of all -- is that your children might get hurt by playing with the cart or climbing on chairs when you were too busy to watch them.
That said, here is one suggestion: to keep them more under control and safe, could you have them help you sort laundry, put it into the washer, measure out soap -- in other words, involved, busy, active, and helping? Hope this helps and hope it didn't hurt your feelings.