My guys are now 6 and 8. We have had a PW50, JR50, and a KLX100. Cam (6 - 54 lbs) now owns a JR80 for a yard bike, and a KTM SX65 for a track bike (long story). Nic (8 - 50 lbs) has just traded his KLX110 for a very modified TTR90.
I don't think you'd find the JR50/KLX50 to have any real advantage over the PW50. Better rear suspension. Not any more power. PW and JR are about 2 hp bikes.
One of our riding friends has the CRF50. It is larger, with the gears is more versital, but dosn't seem to have any more power (than PW/JR) stock. With these you can BUY as much horse power as you can afford. I don't think the suspension is really any better stock than JR, but then again with MONEY....
The "full race 50s" (KTM pro-JR, Cobra, ...) would have much more hp (I think the top of these run near 14 hp, or at least claim that). They have much better suspension. They need MUCH more maintanence. MUCH! Those little auto clutchs need constant attention. KTM 50s come in Air Cooled (Adventure series) and water cooled (Pro series), both short (JR) and tall (SR). So small with power is the Pro-JR. New I think Pro-JR would run over $2500. Alot more bike than a PW.. but alot more cost (both up front and then for ever in maint.). Also full MX bikes are quite loud for yard use.
So it really depends on use and the child. My 6 yr old LOVES his KTM 65. He had distroyed the shocks in his PW 50 by the time he was 4 yr old jumping it. His JR80 yard bike dosn't thrill him any more. The suspension only survives because the jumps in our "yard track" are not large. He over revs it (trying to get KTM HP out of it), so we do top ends at least once per year (at about $50 each, who cares).
My 8 year old rode his JR50 till he was 6. Shocks were in perfect shape when he sold it(seems to be about the same level as the PW50 suspension). He loves his TTR90. His favorite feature, electric start. He does not ride Cam's KTM 65. Has no interest in that. He like to trail ride.
So, if your boy is more like Nic; buy the CRF50, and with hop ups you will get many years out of it.
If he's more like cam, sell your kidneys and get the KTM Pro-JR. Learn to enjoy lot of quality garage time with the little guy and the little auto clutch :-).
Which ever, have fun.