If you want something kid-friendly without diving into full ecommerce complexity, a couple of lightweight options might work well. For junior-age kids, I’d avoid anything too “business-heavy” and go with a simple catalogue-style setup. Shopify Starter (catalogue mode with payments disabled) or Square Online can both be stripped down so they only display products and collect expressions of interest, not money. They have clean dashboards where kids can upload photos, write descriptions, and “manage” pretend orders.
If you prefer self-hosted, OpenCart or PrestaShop can be configured as catalogue-only and offer user roles, so the kids can handle listings while you keep admin control. They’re more flexible and avoid the WordPress route entirely.