I'd suggest some kind of keepsake nice box / Jewellery etc rather a toy. What about a a name plaque for their room, you can make one easily out of Lego pieces superglued onto a Lego base, it's a really nice way of you giving your child 'permission' to settle in their new home and this is an incredibly precious thing you can do for them. I definitely wouldn't record a voice message as from an adopters perspective I wouldn't have felt happy giving my son a toy with a voice recording, eventually it will run out and that could unsettle a child and not all adopters would be comfortable with it.. Take care of yourself
I have decided on a necklace each which says "I love to the moon and back" for the girls to go in the memory boxes. I wasn't comfortable with a voice recording. I also was concerned that if I did a voice recording the adopters wouldn't let the girls keep it.