Extremely hot story, very kinky :)
Just start off casually telling him look what's I found in our girls room, see how he reacts and go from there
Let us know what happens
I think you should approach your daughter first. Let her know you found your dildo in her room, but you're not mad about it. Then casually mention that it's special to you because it's made from a mold of your husband's cock.
If she shows interest or asks questions, ask if she'd like to try the real one if you can talk her father into it. Alternatively, you could give her some pointers on seducing him and proceed without him knowing it's a set play between the two of you to get him to fuck her.
If he comes to you with concerns about her suddenly becoming blatant in showing off for him, you can tell him she's horny and he should take advantage with your blessing.
Of course, if he goes ahead and beds her without telling you, she needs to give you all the details and you need to share them with us.
Tell your daughter you know she is using your dildo and it ok to share anytime, ask her if she knows its a mould of her dads cock and if she does ask her what she has in her mind when she is using it, is she is thinking of her dad is she imagining its her dad that's doing it, I use to do the same when I was young thinking it was my dad in me, nothing ever became of it even though I wanted it to, I would like to hear more.
Hell yea tell him i want to here more;)