4-H Nationals follows the 3PARC (CMP "bluebook") rules so if your kids are going to shoot any sanctioned 3PARC matches the only sights allowed on a sporter gun are:
1) Those that came with the stock rifle (i.e. the 888 originals)
2) El Gamo "type" ... whether the Spanish versions or the Chinese "look-alikes".
Daisy has the Chinese ones now and the quality has gotten much better over the last two years. I've not had any issues with the one's I've received in 2008 or 2009.
It appears to me that the Crosman Precision Diopter are "Gen2" Gamos.
(from the picture on Airgun Whse's website)
Original Gamos had an embossed area on the top of the rear sight that said "Gamo" ... "Gen2s" have the embossed area but no "Gamo" print in it.
The Chinese have no embossing on the top.
$89-$109 is what you should expect to pay for the Gamo's, and $35-$45 for the Chinese.
Whatever you do, don't go with the AirForce Edge sights ... currently they are ONLY legal on the Edge. That may change, but the council has not yet allowed them.
Note this section of Rule 4.2.8:
4.2.8 Sights
Only sights manufactured for and sold with Sporter air rifles are permitted. Sights not manufactured specifically for an approved Sporter air rifle (see Rule 4.2.8) are not permitted. Sights for the Daisy M753/M887 air rifle (El Gamo-type) may be used on M853/953/M888 air rifles.
NRA sanctioned matches are a different matter, but again, 4-H uses the CMP rulebook at Nationals.