The unit is the family, not the attendee
Conference maths assumes one or two people per room and roughly one room per registration. Youth sports breaks that assumption completely. A player travels with at least one parent and often with siblings and a grandparent, so the room count per registered participant is higher, and the shape of what those people want is different.
Families also travel together on purpose. Several families from one club will look for one house rather than five rooms, because it is cheaper per head, the kids are together, and somebody can cook. That decision is invisible to a hotel block and it is extremely common.
Sizing a tournament block
Start from participants, multiply by realistic travelling party size, then divide by people per room — and be honest that a family of four in one room is normal here in a way it is not at a conference. The result is the rooms the tournament creates. The share of those that flow through a block you contracted is smaller than organizers expect.
Contract under that share. The attrition risk on a youth tournament is real precisely because the demand is genuine: the beds get booked, they just do not all get booked in your block.
When the block sells out, everybody loses
A sold-out block reads like a success and is usually the most expensive outcome available. The rooms you contracted are full at the rate you negotiated, and every family who booked after that point paid rack rate somewhere else, with no rate benefit, no proximity guarantee and no help from the organizer.
In a real event market we measured, about 31% of the lodging money went to properties that were inside a block that had already sold out — which is to say the demand kept arriving long after the block stopped being able to serve it.
What organizers can do without holding more inventory
The answer is not a bigger block. A bigger block is more attrition risk on a demand curve that nobody can forecast a year out — and roughly 70% of the lodging money in that market was committed inside the final 30 days.
The alternative is to answer the question at the moment each family asks it, with whole properties and rooms near the venue on the tournament dates, on the page they are already on. Bookings carry a 15 to 20% commission, there is no inventory to hold, and the organizer earns on beds they were never going to contract for.