First of all, from a monster perspective, it was NOT "kidnapping". Monsters did not consider humans, let alone human children, to be people, and by definition, kidnapping is taking and holding a PERSON against their will or without their consent, or consent of their guardian in the case of a child or someone unable to give/deny consent. Monsters considered human children to be just animals, as we ourselves think of dogs, rabbits, Guinea pigs, turtles, etc. Human children were a resource and very useful, but were also viewed as toxic, disgusting creatures by their entire society, not just by a select few monsters.

But to answer that question-and understand that this is inference-as to why Randall chose THAT particular child, I honestly think that he was "hedging his bets" with regards to the Scream Extractor actually working by choosing a kid he knew was already terrified of him and who would therefore scream reliably no matter what. That would allow Randall to use the lowest possible setting on the machine, the safest level, and still get results, enough to hopefully please Waternoose and end Randall's part of the bargain. He'd get his promised reward(yeah, I'm sure he would have), whatever Waternoose had promised him in exchange for building and testing the machine, get Waternoose off his back, and once his boss secured official approval for the machine, he'd be famous as its inventor and savior of the economy and finally get the respect and acceptance he'd wanted all along. In the process, he'd by-pass someone he believed was still cheating and riding a famous family name to success, James P. Sullivan. If you can't beat a cheater at his own game, invent a new game. Most people didn't even realize that Randall only had the machine's control panel set at 15% when Boo was strapped into the seat, and that Sulley pushed them all the way up to 100% when he put Fungus in the seat, so they assume that the same thing, or worse, would have happened to Boo that happened to Fungus. Another reason why Randall chose Boo is because he did NOT, contrary to what some people believe, design the machine to kill or permanently harm human kids. What would be the point in doing that, knowing that human kids, disgusting or not, are still vital to Monster World economy? Randall intended to simply return her to her room after the test, and didn't want to pick a kid old enough to verbalize what had happened to them once they got back home. They could not describe what they'd seen to anyone, so it was necessary to pick a child that was too young to really talk just yet.

great explanation, however, the scary crabby monster said himself that he’d ‘kidnap a thousand children’, yes they thought of children as we think of animals but it was still acknowledged as kidnapping 😁