I just picked up a 20 gauge Remington 11-87 Sportsman Compact for my son as well as a small Mossberg 510 in 20 gauge.  Both are nice sized for kids.  I just don't care for the front bead and want to change them out for a fiber optic type front bead.  My question is do I just grab some pliers and twist it off or is there something I need to do with more finesse?  Are there any standard thread diameters?

Thanks,

I don't know about the Mossberg 510, but on the 500/590/835's, you can use a small wrench, and just unscrew it.
you might want to look at the Meprolight Tru-Dot for the Mossberg.
http://www.opticsplanet.com/meprolight-tru-dot-shotgun-night-sights-for-mossberg-shotguns.html