How can seedless watermelons make baby watermelons?

Good question. It's a complicated process that involves crossing two diploid melons to get a triploid. At bit like crossing a donkey and horse to get a mule.

And like a mule a seedless melon is unable to reproduce so in order to get another seedless melon you need to repeat the process.

Here is an article that explains the process more in depth.

Thanks for your interesting question.