oh brother!!! that's too bad for them!!!
or you can make a compromise with them??? like some can wear flipflops and others can wear sandals (make sure they match and blend in) and so forth... ]
my best friend was very smart not to have bridesmaids in her wedding cuz it was too much of a hassle... i plan to do the same too ... yikesss... i don't need the stress.. i saw what my sister went through... FORGET IT! 
i was a bridesmaid in a wedding.... i hated the color and design of the dress the bride had designed for us... so she ended up paying for the dress and changing the design of the dress to fit me... (i was heavier back then) as the the dress was designed for thin people sheesh (strapless/tight, not full if you know what i mean?) i felt very uncomfortable..... and the color sucked too as it was her favorite color back then!!???
(brass-gold color!
) some of us couldn't believe she picked the color... but she did it anyway... cuz it was HER WEDDING...
another cousin who got married a few years ago wore white tennis shoes with laces cuz she hates pumps/fancy shoes and so did a friend of mine... (we didn't even notice it until they were dancing)... their parents couldn't say anything cuz they had their "last words!" 
so my point is... IT IS YOUR WEDDING!!! PERIOD! tell them they can PLAN THEIR OWN WEDDINGS AND YOU WON'T INTERFERRE WITH THEIRS! PURE AND SIMPLE! 