Hello = ) - Quick question; are you measuring the 50 gallon by L x W x H? The species you have listed are all terrestrial, so floorspace is going to be much more appreciated than height. If that enclosure has an 18" height, you might want to consider getting a 40 gallon breeder enclosure (bare min for some adults) or preferably something int he 65-75 gallon range with a 48" x 18" footprint. The bigger then better, and the extra width is sometimes appreciated by larger adults...I've seen large dragons that can't easily turn around in enclosure with a 12" width (although it is doable).

Blue-Tongued Skinks:

1) Diurnal

2) This depends entirely on what you are accustomed to and what you would describe as "fast." I believe BTS's are fairly slow compared to some species of gecko that can be quite spastic. My limited experience with them has shown them to be pretty laid-back and relatively reluctant to do anything speedy...although thats not to say they aren't capable of being fast.

3) Yes, although adult supervision of care may be required. Be sure the child is responsible and regularly changes water, feeds, supplements, and cleans after the animal.

4) Yes - this species is known to tolerate handling quite well. Gentle, regular handling sessions can result in a "tame" animal, particularly adults.

5) One.

Good resource for care on BTS : Detailed Blue Tongue Skink Caresheet - BlueTongueSkinks.NET

Bearded Dragons:

1) Diurnal

2) Juvenile beardies can vary in tempermenent and speed, adults are usually very laid back and hesistant to react violently or with flight.

3) Debatable. They make great pets; possibly the most personable lizard out there. They truly form bonds with their owners and can recognize different people in the family, make associations, and I believe they actually have some reasoning skills. That being said, they are great on a personality level, but they can be a handful to care for. Juveniles require immense amounts of food, and both adults and juveniles require insect and plant matter to survive. They are a higher maintanence reptile.

4) Yes; particularly adults. I truly believe Beardies actually enjoy being handling and receiving stimulation.

5) One - although a slightly larger enclosure might be appreciated by a large adult.

Good starting place for care information : http://reptilerooms.com - > Caresheets - > Beared Dragons (not allowing me to access our site for some reason right now)
Bearded Dragon . org - Bearded Dragon Care Sheet <--- Another Basic Caresheet
Nutrition Content <-- List of salad items to feed, and frequency of feeding. Good info site overall as well.

Reptiles as a general rule are not social creatures, they prefer to be kept alone and do best alone. The 50 gallon you mention is bare mimimum size for a single adult animal of either of these species, so more wouldn't work anyway. Its always best to house your reptiles alone. Be sure to adequately research the care of whatever you choose. If you have any further questions, I can elaborate further. Just gave EXTREMELY brief info that you asked for. Ask away if there is anything else you'd like to know, I can provide more good care information links as well (a lot of stuff out there is junk).

I'm not as familiar with crocodile skinks, sorry = (