i commend you on your bravery. doing individual cakes is time-consuming and challenging, so bake your cakes and freeze them before decorating, and give yourself LOTS of time just to decorate (at least 5 hours -- estimated at 10 minutes per cake, but you might need more). i've never used the mini-tier pan, so the advice i'm going to offer is very general. question: are you putting them on little plates or little cake rounds? keep it simple. set yourself up for assembly-line style work: lay out several cakes at a time, ice them all, and then decorate. make a few extra, in case of accidents anywhere from the kitchen to the bridal shower.
for such a small cake you will want to keep the decorating simple, like flat-iced buttercream, with maybe a tiny pearl border (use round tip in size 3, 4, 5, or 6) at the base and top of each layer (work from the top down). if you don't already own a small palette knife (spatula) run out and get one, as well as a small pointed off-set palette knife (this comes in very handy, especially with decorating small items). you might want to finish the top of each cake with a rose and some leaves (and perhaps some trailing vines over the edge), but remember that whatever you choose, you're going to have to do it 25-30 times over!. a better idea would be to make a little topper for each cake by cutting a cookie into halves or quarters and sticking it on the top by pressing into the cake or pipe a rosette on each and lean the cookie on the rosette, so that it stands up straight, giving you a fan effect. good cookies for this would be a pizzelle or other wafer cookie, or you could even cut up a cynlindrical cookie, like a pirouline (a wafer cookie, rolled up into a cigarette-shape, and filled with chocolate or other cream), and stick one or two into the top of the cake (if using two, have one taller than the other). of course, if your cousin loves oreos or teddy grahams or miniature chocolate bars (etc.), you can use those too.
tips for mini-cakes: apply a dab of icing to the plate and set the cake on top of it to help secure it to the plate. you can then hold the edge of the plate while you decorate, or, you can insert a drinking straw or slender dowel into the top of the cake so you'll have something to hold onto to steady the cake while you ice the cake with your other hand (be sure to remove it and ice over the hole when you're done).
best of luck and let me know how it goes.