No, they are no where near as bullet proof as the PW50. But performance is at a much higher level.
The chain, sprockets, chain guide, and chain adjusting hardware on my sons 98 KTM 50 pro Jr are junk. Be very careful tightening the chain adjustment bolts to not strip them. The chain stretches daily. The chain guide bends every other day.
The airfilter gets dirt VERY fast. It is snaller than a 6 year old's sock.
The jetting on mine was very rich. I needed to take the main jet down a few sizes, even at sea level.
I do not have that much experience with the adventure. It is basically the same frame except for the suspension as my son’s 991/2 Pro JR and 00 Pro SR. There is a big difference in the engine and exhaust though. I do not know your nephews age or size, but depending on what your nephew plans on doing with the bike you may have been better going with a holdover 2000 Pro SR or if available and a little extra money the 01 Pro SR. But for general riding in a field or some gradual hills it will be fine. It just does not have the snap or get up and go as the Pro bikes.
Keep an eye on the coil gap; KTM's are historically noted for vibrating bolts loose. Red loctite everything that you do not plan on doing regular maintenance to and blue loctite everything else. The brake levers are a little big for small hands but if he is into a SR they may not be that bad they do make a rear foot brake assembly for about $70 if it is not already on the SR Adventure.
The stock sprockets were not on the bikes that long as they were changed for gearing with the chain. I have not had a problem with the chain guide or adjusters. Watch the chain tension keep it adjusted, as it will put unnecessary tension on the counter shaft. The air cleaner under the seat is small and does get dirty easy and choke the bike; there is an external pod filter available.
If you are going to do maintenance your self there are a lot of brains to pick here http://www.holeshotktm.com/ as they are strictly KTM and have a mini's forum I wish DNR did.