Most probably - everyone should receive a boon, but still worth to clarify:
"...GMs should carefully consider
which PC is best suited for this
responsibility..."
and
"...Accepting the pseudodragon grants the PCs the “Draconic Sidekick” boon..."
Does it mean that only one PC in a party receives that boon?
Or everyone receive it as usual?
If so, - what's the point to GM to "consider carefully" who should receive that dragonling?