I’ve been looking for some plain, long sleeved tops for my daughter (the sort of thing she can wear under a pinafore dress and can get mucky without it being a problem). This used to be the sort of thing that John Lewis sold (like plain leggings) but they seem to have stopped. In the end I found them in Next.
I don’t really like Next but they do seem to have quite a few basics for children and are not expensive. I used to have an online account with them but closed it after my account got hacked so have to buy stuff in person. Which is a pity but not the end of the world.