I've just been reading Josephine Pullein-Thompson's Plenty of Ponies, which is one of her earliest works. It's the story of the Esmonds, whose parents have, contrary to usual pony book form, hit good times, meaning that they now have a pony each, a large house and plenty of staff. Lewis says: "It's much easier to be nice if you're poor." Charlotte, his sister, says "I should think it's extremely difficult to be nice if you're really poor and don't have enough to eat and live in a slum, but I agree that it's good for one to be poverty-stricken like we were."
I wonder if it's the idea of noblesse oblige that JPT likes: it's alright to have money, but you must do the right things with it: treat your fellows, staff and horses well and live a life that doesn't focus simply on what you can buy. The main adult figures in the Noel and Henry books, Major and Mrs Holbrooke embody all these things. June Cresswell, on the other hand, has plenty of money, but also a mother who is wilfully blind to her daughter's faults and determined to believe only the best of her in spite of all evidence to the contrary. The heroine of I Had Two Ponies is a classic spoiled little rich girl: so used to having her own way, and doing nothing for herself, that she is not particularly bothered when her two ponies are sent to a sale. She is, however, shown the error of her ways, realises that she has been behaving extremely badly and hunts out the two sold ponies. Two Ponies is, I think, the most moral of JPT's works (do tell me if you think differently) and has the character who goes on the greatest moral journey.
I've been trying to think of a pony book in which the heroine is poor and nasty; ground down by her poverty, but I can't. The most realistically portrayed poor girl is Ruth in K M Peyton's Fly-by-Night. Dinah, in Joanna Cannan's, Gaze at the Moon lives in a Council house. They are both poor but have loving families and masses of determination and a good deal of self-belief. Jill, of course, has a mother whose income fluctuates, certainly in the early stories, but matters never become dire. Perhaps that is anyway, one of the functions of the children's story: to act as a fantasy, where the worst never quite happens, or if it does, it's overcome.