Eating watermelon seeds will not make watermelons grow inside your stomach so no need to worry.
My parents used to tease me about that when I was little and I used to be scared of a watermelon growing inside my stomach but no watermelon has ever grown in my stomach in all those years.
So if you swallow a watermelon seed the only thing that will happen is that you'll poop the watermelon seed out in your poop when you digest the watermelon seeds.