Here's an issue that needs honest clarification so that consumers can avoid the deceptive marketing that prevails. A low frame tube on a balance bike is often sold to parents as some kind of advantage. After all, the bike will be easier to mount and dismount, no? No!
The only time a toddler pokes his or her leg through the bike's frame to get onto it, is the first few times they ride it. On the 2nd or 3rd day, kids intuitively discover that it's much quicker and easier to swing their leg up and over the back wheel, and onto the saddle. This is how adults get onto their bikes and motorcycles, it's how cowboys get onto their horses, and it's how kids get onto balance bikes.
Conclusion: A low swooping frame tube is just a marketing gimmick that targets folks who don't usually ride a bike and don't know better. No matter how the bike's frame is shaped, the top tube will always be lower than the saddle. Marketing is very good at solving problems you don't even have!