For very young kids, pedal thickness and width matter more than platform size. Many standard pedals are quite thick, which can make it harder for small feet to stay on.
Junior or slim pedals are usually easier for beginners because they’re thinner and narrower, helping foot placement and comfort. Surface area isn’t critical at this stage, so if you can get slim kid pedals, they’re often a better choice for a first uni.
A search found some pedals marketed for riders with small feet on OneUp Components: “Small Composite Pedals” for ~$50 US. There was also another hit on unicycle dot com for DMR V6 pedals. The OneUp page did not say if it was 9/16” thread or 1/2” thread … but I assume 9/16”
Is it a relatively modern uni with standard sized pedals (as in the size of the thread that goes into the cranks). Most pedals now use a 9/16” thread but some old/wierd unis might use 1/2” threads. Hopefully not going to be a problem but worth mentioning…
Three options really
Local UDC equivalent - something like this from UDC UK.
Friendly local bike shop - especially one that stocks a decent selection of kids bikes - for a child learner, there’s going to be no difference. Possibly your best way to get specific advice if you aren’t certain on anything.
Amazon/local e-commerce platform of choice - they might have better prices but they cannot provide advice and the quality will vary wildly.