You mentioned a "blanket box", how would an ottoman work its the same thing but phrasing is cheaper if on a budget like many of us are
example www.argos.co.uk/product/9040229
I have a couple of these ones myself and when needed the have become temporary seats for 2 kids / 1 adult with loads of room -
takes seconds to assemble, take out of packing, lift lid away from main part, expand main part out so it is standing up, put "floor in it!, put divider if wanted in it,
add stuff to it and add lid to hide whatever, depends on the age of your DDs but you could cover the top halves of both sides of the ottoman with towels and bedding and stash a layer at the bottom of goodies, to be raided when had a bad day etc
Depending on your finances etc I used to find this was a great place to stash a couple of extra tins that i bought each week with weekly shopping as didnt notice a £1 or 2 more and had an emergency stash ready for a time when things got tight, my ottoman saved me