I am wondering the same thing. Mildred is on our list for dc5 (and is dh's favourite), and we are not sure whether to use Dilly from the start, to nip Millie in the bud straight away, or to just try and stick to Mildred (I love Mildred) I can't really think of another nn for Millicent though. I have never met either in rl.
One baby Millicent, nicknamed Millie. Lovely name IMO. I do think Millie is the most natural name for Millicent, and a very pretty one. If popularityof Millie is the problem I think probably better to choose another name.
No Mildreds. I don't like the "dred" ending and it's a bit wet.