There are few “quality” kids bikes around that are in a price range most parents are willing to venture into. It seems that even regular riders have a hard time justifying spending more than $200.00 on their kids bike even when their own bike cost $1000.00 plus. It can easily cost more than $200.00 for a quality set of wheels (the wheels on my son’s BMX race bike are $250.00 and they are only mid-level) so most affordable bikes will have a lot of compromises. Even when you are willing to look into the higher ranges you have few real quality choices in 20 inch wheel sizes and only a few more when you hit the 24 inch wheel size.
As for the bike in question. The red plastic sprocket (they are called jockey wheels) is going to last just as long as the black plastic jockey wheels that most bikes come with. The derailleur is the ubiquitous Shimano Tourney derailleur that comes on practically every kids bike with gears. The only thing that makes it look flimsier than prior models is that the jockey wheel is much larger than what most people are used to seeing on a derailleur, this was a change in the Shimano low end component groups about 3 years ago. Google Giant STP 225 and you will see that for 350.00 or more you get a bike equipped with exactly the same derailleur. If the derailleur does pack it in buy a new one and throw it on. A new Tourney derailleur can be found for under 10.00 and you can move up to an Altus level for 15.00, it literally takes 10 minutes to do this. The Pro-Max brakes are fine if set up properly, though they usually are not. The bottom brackets are usually cheap heavy ones but they should last unless they were threaded in improperly. The headsets are usually terrible but are strong enough for the type of riding most kids do. The wheels are generally low end machine built wheels that rarely have the spokes properly tensioned, they may or may not stay true, it depends a lot on how the kid rides.
Bikes typically come in a box with no pedals on, the wheels not installed, the handlebars turned sideways and the seat not in the post. The bottom bracket will already be installed as will the crank. There is very little actual assembly to be done so there is no reason to believe a big box store employee can not manage this. The problems tend to arise as a result of the bike not being tuned after assembly. I would suggest you point the parents at the Park Tool website (www.parktool.com/
) repair help section. They have easy to follow instructions on how to tune the brakes and shifting, these are tasks that can be accomplished with a few allen keys and a screwdriver. If they are handy enough to tune the bike themselves then it will be fine.