you are too naive, they say a tiger won't eat it's own cubs, but humans are different. We are selfish creatures and there are humans that will eat their own flesh and blood to survive much less sell a child for comfort and convenience. They believe they can always have another.
Bull animals eat their own babies all the time they don't care in the wild it's the law of the jungle and the need to survive, it's also about needed sacrifice. On the other hand humans aren't just selfish they also have the unfortunate double edged sword of intelligence and due to that being used for evil some are forced to do things they never wanted to, i'm not saying Iruma's parents are good (since honestly we don't really know?) but in the real world parents have been forced to sell off their children not just for their own good but in the hopes that their children will have better lives too, i mean right now young girls are 'married off' not just to help their families but in the hopes they will get a better life too. Sorry got a bit full on there but mostly the animal part made me laugh cause i literally have pet fish who eat their own babies just because.
i don't know if people already got this but what if the parents was prepairing him for this because i was reading the raws and i was like what if the demon was already in contact with the parents and they made a deal because i don't think any parent would put their child in danger for no reason or they could have just been that stupid