The gift shop at Riverside will have books and pens but they do a few so they might like to choose?
If you get an early breakfast they let you through the turnstiles before letting you down main street and there is a shop to the left that also has books & pens.
I would also let them pick their own ears - there is a whole shop in MK full of different ones.
If you are getting the PP+ you will not need that for CP - there is no photo package there so if you want photos taken on the way to breakfast just grab a normal card off a photographer on the way and head back and collect it after breakfast and they can add on the photos from the other card as well if you want.