This is another one of those "how long is a piece of string" threads.
Your cars ar eworth what someone else wants to pay for them.
The 959 was in fashion some time ago and may come back again, but it is the shell that is worth the most not the rest.
A so-and-so shell (say it has cracks) is not worth much as there are reproductions available these days which are getting better and better with each generation (and they're certainly more durable than the original).
An original shell in box basically can be sold for the asking price if the right person comes along. Then again, if it doesn't....
A restorable shell is worth about the price of a new repro.
This is all very general and basically what I have seen since I started buying these cars (I have two now and enough parts to keep them running with a little bit of care, but that's it, not buying anything any more).
There are a lot of inbetweens, of course, say a shell someone only sprayed some paint on, put it back in the box never to touch it again. That would probably be a quick resto job on a basically untouched shell. Things happen. Whoever finds one has a good chance of paying a low price for it. But these things are rare and getting rarer every time another one is dug out.
Plastics are pretty fragile and more often than not on cars actually used, the front gearbox cracks at the point where it attaches to the chassis. This is a major problem and can only be fixed by a replacement gearbox.
So you see, the actual value depends a lot on the actual state of each individual part in the box, because that's what you are selling now, a collection of psare parts, not a complete kit (even though you may think it's a complete kit). You might actually make more money breaking it (sad, but true). Some items like the original motor, tires, wheels, hubs, are things that can make or break the value of your car as they are nigh on impossible to find (and can't be replaced by modern items as nothing fits straight on).
So my advice is try selling it somewhere where collectors gather. They'll appreciate what you have, but be prepared to work a little bit to get the best out of it (i.e. describe it in detail for your potential buyers). Tamiya club is a good place to try and advertise.
For a kid (and a five year old at that) this is going to be worthless. It will be trashed instantly. Buy him a decent chinese piece of cheap crap that will see him through childhood easily.