This is an excellent starting point. I applaud you for already putting your imagination to work. It’s exciting isn’t it, that feeling when you have a little kernel of gold in your mind that you feel sure will turn into something wonderful?
I’m afraid having the idea is the easy part. Building that idea into something takes work. But you’re prepared for that, right?
Ninety-nine time out of a hundred, an idea will wither on the vine, not because it’s a bad idea, but because the writer doesn’t know how to nurture it. When the first flush of excitement passes, you can’t see where it goes next. You struggle to visualise the span of your book. You can’t make your characters sound the way you want, or look the way you imagined. Your story only stretches to one page and you don’t know how to fatten it up. This is as true for experienced writers as it is for those just starting out. I would hazard a guess that every published author in the field of children’s books has a drawer full of unfinished stories and half-sketched ideas that simply didn’t go anywhere. You are in good company.