Estimate how much money one could make by writing a fictional children novel of only about 100 pages or so.?

It depends how well it sells, really.

If it doesn't sell at all, you won't make a penny. If it sells a few thousand copies, you'll make an amount in the low thousands of dollars.  If it's a bestseller you could be looking at hundreds of thousands, especially if you can get a good deal on film rights and sales in foreign territories, and ideally win two or three awards.

If you're hoping to be the next JK Rowling, take a reality check.  Her level of success is a once in a century fluke.

Zero, if you don't have a publisher, an agent to interest the publisher and represent you for the contract, and if you have no money for advertising and marketing, and of course, if you have no talent.

That would entirely depend on who "one" is, on that person's level of talent, on how appealing, engaging, and interesting that person's story might be, and on whether or not the publisher happened to be willing to promote and market the book effectively enough to move a large number of copies. 

The number of manuscripts submitted by authors looking to have a children's book published is staggering. 

The number of manuscripts that actually get picked up is incredibly small.

And the number that go on to do well is infinitesimal. 

I estimate that you'd be several dollars poorer because you'll never recoup what you shell out for postage. 

One would make nothing unless one could sell it to a decent publisher.