it's easier to have them in the colors of the dots... and depending on the type of dress you have, do either swiss dot for accents on it... or simply have dotted ribbons with it, to match the colors they wear.  

also, try button bouquets for them, as they can take on a more dotted look, or use fewer and smaller flowers in a sea of greenery or baby's breath... lowers the cost of the flowers, and makes them more dotty looking! 

beautiful..before i wanted my motiff as polka dot, but its hard for me to find the dresses for my girls.