This is really horrible for you. I don't know what will be achieved by your DH talking to his ex and his dd about it - they are unlikely to admit it now if they have got it and confronting them will just back them into a corner. The best thing would be for him to explain to his dd how upset he is about it (it does have her prints on it too) and that if it does turn up it would be great to have it back.
As far as getting over this goes - however much it means to you it is just an object. It could have fallen off at the supermarket or anywhere. I am guessing the upset is worse because of the circumstances and the feelings it brings up to do with his ex and his dd.
You have to decide to get over it because if you don't the bitterness and anger will just hurt you in the end. It is not much consolation I know but people who behave like this are generally unhappy and actions like this just add to this state of mind. You are with your DH, you have your family. Don't let them spoil it for you. There is little your DH can achieve by confronting them so just make up with him - he is sad too.
Feel so sad today I feel sick. About 5 years ago I gave DH a 40th birthday gift of a silver fingerprint necklace with the kids prints on - my two DD's, his DD and our DS who was only a baby at the time. He has worn it constantly, but recently the clasp was loose. He should've got it fixed but didn't and it fell off when he was dropping his DD (aged 14) home at his ex's, quite a few weeks ago now. His DD called to say that she had found it in the driveway and would bring it back. But now it has 'gone missing' My stepdaughter claims she has returned it - she hasn't. My DH's ex absolutely hates me and would do anything to get at me. Practically every single gift that I have given my stepdaughter over the years - watches, coats, earrings, lovely things, have gone missing at DH's ex's house to the extent that we asked my stepdaughter to keep things at our house. That has been upsetting enough but this is irreplaceable. I have had a huge argument with my husband as he has let this fester without actually asking his DD or the ex where the necklace has gone. It hasn't disappeared into thin air! I don't know if it is his ex that has 'lost' it or my stepdaughter - she was very very jealous when our DS was born and things have never been entirely right since, she hates having to share her dad and will do all she can to manipulate him, encouraged by the ex. What an earth do I do? We are going out for our wedding anniversary tomorrow and aren't even speaking at the moment! Should I confront my stepdaughter or his ex myself? I can't believe anyone would be so cruel as to 'lose' something this precious and I find it so upsetting
P.S I do realise that it is just a necklace and people have much greater problems! But it is very precious and it breaks my heart to thing it is probably gone forever, especially in such a callous manner