there are many small form factor computers that can fit full size graphics cards and power supplies, one of the cheaper ones is the coolermaster elite 120 i linked to before. It typically goes for ~$50.
the anandtech review also has some tips on assembling in small cases.
it can get tricky building in small cases.
m-atx/m-itx typically have 2 ram slots, which should not be a problem since 2x4gb and 2x8gb kits are not that expensive anymore
do NOT buy ram with large heatspreaders. THEY might conflict with the CPU HSF. If overclocking, you might be better off getting a contained liquid cpu cooler (corsair h, antec kuhler, etc) since they save space around the cpu and the radiator can be mounted elsewhere.
there are some thermal constraints with smaller cases, so you might not even want to overclock.
I suggested a lian-li pc354 (link in previous post) if you do. They are rather large cases, so you can use them as small tables to put stuff on (no drinks though, since there are fan openings

and nothing too dense since the case isn't too sturdy) the older versions such as the pc351 tend to go on sale every once and a while and are a bit wider and not as deep and don't have top fan openings
a lot of m-itx/m-atx boards come with built in wifi. they also might come with m-sata slots which can fit m-sata SSDs. They can help reduce cables/clutter and allow you to mount other drives if you need them.
otherwise, it's like building in a small cramped case. Small fingers may be helpful

when sticking stuff into it, you might want to get your son to help you

.