Ok, so you're suggesting a 1 foot tall plastic/metal mechanical toy that can move, but also can have the legs/arms/head removed and replaced at will?
Starting with the joints, holding them together with magnets should be relatively OK, they won't need to be too strong as the mass of the toy will currently be assumed to be less than 1-2lb. Balance is going to be the biggest issue, you could design the weight distribution of the legs/arms (they always look pretty similar to me) so that on their own will stand upright, then keeping the rest of the body/head to a minimum mass might be enough, but walking will probably be impossible.
Another method could be installing gyros into all of the limbs and then when there are two limbs in place as legs, the gyros could be active allowing a fair degree of stability that might enable walking. One downside of this is that you would need 4 pretty hefty gyros to be able to stabilise the design, hence a high cost.
As for movement, the hip/shoulder joints can be internal to the chassis, knees/elbows could be articulated from inside the feet/hands to keep the centre of gravity as low as possible, and ankle/wrist articulation could be used to increase stability when walking. As for communication, the CPU could be in the chest, and communicate and transfer power to the limbs through an Apple MacBookPro style magnetic connector. This would enable each limb to understand where on the body it was placed. Assuming the joints are approx ˝ inch would allow for a ring magnet, locator pin and 4 channel data/power (USB style) connectors. Batteries should ideally be in the base of the chassis, but on such a small toy you might only get 5-10 mins of movement.
These are just some preliminary thoughts based upon the look of some of Jeph’s more recent drawings of Pintsize close up, if anyone has ideas to improve upon this I’d love to discuss.