I take my kids (DS11, DD5, DS3) raincoats. I roll them up tight and secure with a rubberband and stick them in a zippy in the basket of the stroller with ponchos for us and one to cover the stroller. If it is planning to rain all day I also take DH and my raincoats. I find the kids do better with the raincoats, and since they are stored in the stroller, carrying them is not a huge deal. I am also not crazy about "disposable" ponchos-seems like a waste to the enviroment. It is also easier to spot my kids and hubby!
One of the best days we ever had was a downpour at MK hardly any lines (presidents week) and we slowed down. Take extra socks if you know its gonna' rain!!! I also read a tip to water proof sneakers, I have never done it, but it seems like a good idea.