Demon's know the fate of people, like their future if they're gonna be the prime minister or homeless and so this is how they choose who to possess and also when.
This powerful demon finds this person who is going to be super powerful and or influential so they decide to possess them and lock themselves in the body to stop hunters or the equivalent from the ring them from this person. They only problem is they went into the body way too young and this person is a toddler, but because the demon locked itself in the body it can't just leave yet it has to essentially find the key. But also because the demon is locked in a toddler it's hardly left alone it's struggling to get the key.
I can't expand on this anymore because just the idea of a super powerful demon being a toddler and being constantly man handled and talked to in a baby voice, then said toddler demon get mad and throwing a tantrum is making me laugh so much I can't type.