Hi - good question! I have actually just been through this as well, with four bridesmaids all of very different heights and ages. My solution was to buy them matching multiway dresses plus matching ballet pumps. I found mine in H&M for £8.99 a pair, and they are actually really nice. I have just said to them all that they have the option either of wearing the ballet pumps all day, or of wearing (their own) heels for the first part of the day and switching to the ballet pumps later. (And we have also ordered the dresses in lengths which will accommodate that.)
My bridesmaids have all been perfectly happy with this. Two of them (one who is around 5ft 3" but is only 18, and one who is not 18 but who is 5' 11" in her socks!) are planning to wear the ballet shoes all day, and the other two (both in their 30s, both around 5ft) are planning on wearing their own heels at first and then switching down when the heels become too much.
This does mean that I won't have any say in what sort of style or colour shoes the two who will be in heels will be wearing (although only because I don't feel that I want to dictate to those two what they should wear, given that I won't be paying for them) but otherwise as far as I'm concerned this is a great outcome overall and I'm delighted - and it has got me out of feeling as if I have to choose and potentially pay for four pairs of expensive shoes which probably wouldn't have suited everyone anyway!
Good luck xx