Sounds like you vendor knows his chook keeping
Never underestimate the cunning of a mother fox with bubs to feed.
2 pekins would need very little free range space, infact an hour or two with you pottering around the garden or the kids playing with them will be fine, even if it is just once or twice a week. Small chooks can also fall prey to hawks or even magpies.
The roost is the dark place where they go to sleep (or roost!) and laying boxes is where they lay.
Is the pen portable? can you move it round the yard to fresh grass? if so, that is perfect, if not, just a hand ful of green leafy veges (lettuce, cabbage etc) every couple of days will do them just fine.
Dont forget though, that the bulk of their food should be pellets/grain/mash, a proper good quality chook mix. Expect to pend $20-$25 per bag, which will last you months lol. Try and find out what your vendor is feeding and get exactly the same brand, they will be experiencing a big enough change without thier food being messed with.
Plenty of fresh water, worm them every 3-6 months and they should be fine.
Good luck, so exciting...