You both missed the post and the point.
There is not a home shirt for a 7-y-o. Not in the shop, not on Seagulls Driect and not in some secret stock held for birthday claimants. The shirt simply doesn't exist as it's not manufactured. The smallest size is 8-10 (see here)
The shirt for 6-7 year...
How bizarre.
My guess would be is that it is to do with the Nike kit manufacturing restrictions. The green kit is, I'm assuming, straight out of the Nike kit catalogue with our badge and sponsor then ironed on, whilst the home kit is bespoke-ish (someone who follows this more closely than me...