Responses
I do believe he actually did it.
Everything indicates to It.
He is seem making what it looks to be a wish for Chiwoo to never leave his side, it's then shown the parents death with water coming up like Magic, similar to shown before Magic effects and he hear the news he's immediately happy. Also, now both Jin and Sunny reaction to hearing about Chiwoo's parents death was to be extremely alarmed, it beings even more suspicion to it. It may have not been intentional but It seems that he did, even if he did, he was still really happy about it.
For everyone wondering, no Hwan had no connection to Chiwoo's parents death, it was just a really shitty coincidence. He was a literal child when that happened, and it also happened before he got magical boy powers and such. That being said he was happy about the whole thing, so that's shitty, but I highly doubt a 10 year old can orchestrate a complex accident like that...