Finleysmaid - the chances of getting flour through usual channels such as supermarkets is probably a bit tough to say the least, the last time my tescos had any was about 8 weeks ago, although I did manage to get some with my on line shop, but in person, sadly, never any there. HOWEVER, I do frequent my local farm shop for veggies and they have started bringing in huge bags of the stuff and we can weigh out what we want. Maybe you have someone doing something similar nearby if you want it, not necessarily for school.
Ive also got some air drying clay leftover from nursery days, (not sure how long that keeps must go take a look). That though, might be a good lockdown activity and if the poppets make something and paint it, it could be a keepsake for their "time capsule" boxes :O)