Not specific to soap, but I spent a few years doing markets with my glass work and a lot of the advice will translate. You've had some good tips already, I'll try not to repeat them.
Firstly, think about which qualities of your soap do you want to emphasize? This will be informed by your target customer, the type of people who shop at the market. For example, are your products natural, vegan, artisanal? Those will attract a different audience from handmade, fun and bubbly... Use these words to think about your stall. Piled high and selling them cheap? Go for bright colours, both in your table cover colours and clothing. Or are they more expensive and sophisticated? Dress your table and yourself appropriately.
The more luxurious your image, the more space you need between products. Dark colours, luxury fabrics on the table and a minimalist aesthetic mean you'll likely sell fewer items, but at a higher price.
Depending on your price points, put your most expensive, luxury items highest on the display in the middle, you want them at or close to eye level if possible. This holds true no matter what your general pricing levels are.
Lowest on the table, and right at the front, you may want some small, possibly brightly coloured, cheaper items to attract the kids. Could be something like a basket of lip balms, rubber ducks, small sample sizes /offcuts (if you don't have many small soaps you can cut a few full sized bars into small bits) and sell them at 3/5 for $x.
It's good to have a few items that are cheaper, even pocket money prices, either for kids or for folks who want to buy something but aren't sure about a full price item yet. This is especially true if your core product is a premium, fairly expensive item. Folks like to feel that they're getting a bargain!
A banner isn't very expensive and can be a good way to bring your branding to the stall. It can go behind you in a gazebo, or at the front of the table if the market is indoors. It's not essential for your first market, but if you'll continue then do consider it.
I like to include a photo of myself working somewhere on the table or banner, to emphasise the fact that they were made my hand, by the person standing behind the table... Perhaps one of you cutting a bar might be appropriate? Or pouring the soap batter?
I agree with the card machine suggestion, if possible. I lost a load of sales at first because folks carry less cash these days, and if they do bring cash to a market, once it's gone, it's gone.