Two suggestions: old Ral Partha stuff and 1/72 plastics.
Are you in the UK? If so, Ral Partha Europe has some of the
old Ral Partha adventurers. These vary a bit in scale: the oldest ones are even smaller than true 25mm as they're 25mm to the top of the head rather than the eyes. Those would be bang on for 12-year-olds in 28mm.I think those very smallest ones are flagged up as such in the descriptions (e.g. the robed cleric at the top of the page), but most of them will be small by the standards of modern 28s. If you're in the US, there's a much bigger choice from Iron Wind/Ral Partha legacy. In either case, have a look at eBay for Ral Partha knights, clerics and adventurers - or just Ral Partha. You might find some old Grenadier adventurers (25mm with big heads) that might work well too.
With 1/72, you can get plenty of knights or men at arms that will look child-sized next to 28s. Your best source is to scour the
Medieval section of Plastic Soldier Review (an almost hypnotic website, as many here will attest).
The
Italieri Hundred Years War set gives you a great cleric on foot and (pony-)mounted (first figure of second row and first of fourth - although
look at first in the third row for a very close match, barring the sword!).
The only problem is that for 1/72, you generally have to buy a whole box - but that will only set you back eight quid or so, and you might find a use for the others (elf soldiers in D&D, perhaps - I think they're meant to be 5' or so and slight).
There might be other things that work elsewhere in that Plastic Soldier link. Strelets figures are uglier and squatter, but that might work them well enough as 28mm children. Have a look at the bottom two lines of photos
here.
Hope that's some help!