Initial thoughts:
For anything like this, you're probably best using a vegan recipe, or at the very least vegetarian. If your usual recipe isn't vegan, you have a bit of time to find one you like.
I would also include an unscented option. Make them distinct, different coloured label/packaging or something, and in a separate bowl/basket. Some coloured and a few plain.
I'd personally over-cater a bit, in case it turns out that no one wants unscented soap, or alternatively more people than you expect want it. So for example, if you were making for 20 people, that would be 40 small soaps. I'd do 40 scented, plus another 10 or so unscented.
You can gift the leftovers to your DIL if you think she'd like them. If your recipe is mild, the unscented soap should be fine for baby.
I might be tempted to find some cute cavity moulds if you don't need too many. There are plenty of pretty flower moulds around, for example. Using a few different but similar moulds gives you variety.
I've not used impression mats either, but in a loaf mould they look fairly simple to use. I've also seen them used as a liner round the inside of a pringles tin or round mould.
In terms of scents, I'd go for a single, unisex scent. Lemongrass is one I think works for all ages and genders. Some of the ocean themed scents are unisex too, though I think they'd likely be less appealing to teens and kids, as they are more complex.
Do you think folks will want to actually use the soap? If so, a simple itp swirl or similar might be best so they're not deemed "too pretty to use". If you think they'll mostly be kept as mementos, then possibly consider a more complex design, depending on your skill level. Now is not the time to try new, tricky techniques, in case they don't come out how you wanted.
As I said, these are initial thoughts! Do let us know what you come up with.