I have a clear perspex stand, a wooden stand and a log slice style stand that are perfect for round cakes of 8-10 inches.
You’re welcome to bring along your own if you prefer but a cake stand isn’t a necessity. The thing to know is that if you use a cake stand, the cake is a lot more likely to end up getting tipped on the floor (with the exception of the log slice as that’s already there) which can be cute but it depends on what you are after in terms of photos.
Cake stands can also be a bit help if your child isn’t a fan of getting messy as they let them sit close to the cake without touching it and I’ve found that can encourage exploration in some cases.
They’re not an essential part of a cake smash photo shoot though so if you don’t fancy using one that’s fine, just bring your cake along on a board (if it doesn’t already come with one) or a flat plat so it’s easy to move around and transport to and from your session.