Founder's Pack content will only be granted to the Xbox account that is associated with the Founder's Pack purchase. 

Founder's Pack content and any Upgrade Packs purchased on the primary account will not be granted to any Sub-accounts. In short: sub-accounts will given Fortnite game access only. 

If the Sub-account holder would like to enjoy Founder's Pack content, they can purchase their own Founder's Pack via the in-game Store tab while playing Fortnite.