You're right, Anya, (oh, and I liked Y6 too, especially the boys at that age!): part time work should be made more widely available. There also needs to be a radical rearrangement of working practices so that people who need the money are able to do jobs that are comfortably and safely within their capabilities as they age. Waiting until they give up because their bodies and minds just can't cope, and then refusing them any benefits on the grounds that they're"fit for work" is simply cruel, and inflicts hardship on people who don't deserve it.
Retail is indeed an option; a relative of mine who works as a manager for a large supermarket chain says they'd rather employ over-65s than under-25s on the tills, as they are more likely to turn up on time and not have mysterious Monday-morning illnesses, and they're often more comfortable making small-talk with the customers.
In teaching, though, as in other child-related work, even if you have the physical and emotional stamina, there's a whole world they have to navigate that older people know little about. Social media, messaging, Snapchat, Whatsapp, Instagram .... then there's "sexting", and online pornography available in forms we never dreamt about. Adult who are closer in age are more likely to be able to keep up with this and know how to educate and prepare young people for the adult life ahead, which will be worlds away from ours when we grew up in the 1950s/60s/70s. Some jobs are best left to the younger generation.