Any kind of robust ball e.g. tennis, small light football, if space allows, they are MAD for football there.
And cars and trucks, epsecially dumper trucks or things with rugged wheels if its a rural area because it will be sandy and rocky all around.
Robust plastic plates and mugs, preferably with different patterns, so they can be their 'own'
Toothbrushes and toothpaste, the more the merrier.
Rag dolls or strong plastic dolls, skipping ropes.
I think jigsaws and marbles are a non starter (sorry celebmum) but the facilities are often so poor and the outside play space will be dirt/sand, and the pieces will be lost/eaten in no time.
When we lived there, and had quite a lot of contact with orphanages, the kids loved UNO (card game)a lot!!
Flip flops - light to pack, cheap in chainstores/pound shop in a variety of sizes..sizing not too important with flip-flops......theirs are probably worn through or non existant.
These kids are likely to be extremely poor, any toy is likely to become communal and used until it wears out and then some to the more indestructible the better.
Or maybe take some cash (are you and your friends feeling generous??) to buy school uniforms, which are relatively cheap (£5 for jumper, shirt, skirt) by our standards, but impossible for some to afford. If they're not in uniform, often they're not allowed to attend High School.
We knew of a family with three sons and one school shirt, where they each had the shirt for a day at a time could only go to school in rotation 
Wow! I hope he has an amazing time.