I’ve used a pond liner for a tiny splash pool before, and it worked okay short-term, but there are a few things to keep in mind. Most pond liners aren’t designed for constant chlorine exposure, so they can degrade or get brittle faster than pool-grade vinyl. Sunlight can also speed that process, especially if it’s a thin liner.
For a very small, low-use dipping pool, it can be a cost-effective option, just don’t expect it to last as long as proper pool liner. If you go this route, try to keep the chlorine level on the lower side, use a UV-protective cover when it’s not in use, and check it often for tiny tears. For anything bigger or high-use, I’d still invest in pool-grade material to avoid constant repairs.