Every large corporation has an annual advertising/promotional budget. They hire ad agencies and PR firms to handle that work. These agencies would be the people I'd expect hiring/paying the influencers. I'm sure they have a group of young fresh out of college employees that do nothing but scour the web for influencers keep tabs on their followers, the type of content they produce, etc. When a campaign comes along, they fit the prodcut with the influencer and hope for success.

I meant from the customer side who is spending the money, because kids generally don't have that much discretionary income.

There are a million children in the USA whose parents are millionaires.

Non-poor American and European kids certainly have influence over how their parents spend money on them - almost all of them have discretion over which clothes and accessories are bought for them, if not how much money will be spent.

Saturday morning cartoons were invented to sell things to American kids (by way of them begging their parents).