Baby Pidgeon

In case, you cannot obtain mixed grain quick enough, any supermarket (Tesco, Sainsbury, etc) sells pine nuts (expensive but very high in calories, highly palatable and easy to digest), wholegrain seeded bread (eg 'Seed sensations' or the one from Tesco's contains a terrific variety of high quality small seed), packets of sunflower seed, linseed, granola-type cereals, chopped-up cheerios crushed or lightly soaked in water (milk is toxic to most animals), oats, unroasted & unsalted chopped-up peanuts, you can also try chopped-up boiled egg, which sounds odd but it is one of the most complete and balanced foods for many wild animals in terms of vitamins/minerals/amino acids/proteins. You could leave small size maize (ie uncooked popcorn), brown rice is ok. Lastly, you could soak lentils, pearl barley, or feed fresh/thawed (not tinned) garden peas, but, as with any animal, you have to remove these types of food within around 3 hours before it goes off.
You could also put a small pinch of salt and sugar in the drinking water as an electrolyte.
I am not very knowledgeable about wood-pigeons but young feral pigeons go through quite a lot of stress during the weaning phase, and their mortality rate is much, much higher than adults in part due to man-made viruses (eg young pigeons disease which began after pigeon racers fed their birds corticosteroids, this has been banned since the 70's). Their parents don't tend to abandon them completely even after they fledge, the parents are usually extremely/over- devoted. A 'father' or 'mother' pigeon will fly from Barcelona across mountain ranges and the English channel, if they have chicks to feed or a mate to get back to. That is how their owners make extreme amounts of money. A racing pigeon was stolen, taken to Romania, escaped and 5 years later got back to its box (with its old mate), having never been to Romania or Eastern Europe. If your observations are correct, then I think something sinister may have happened to them, but if the youngster has fledged it may not be at too much of a disadvantage, as it will learn from other birds.
For reference, a young feral pigeon needs on average about 10pct of its body weight daily to survive, ie 30g, or a handful of seed-type food, not sure about a woodpigeon.