The challenge you do create for them is lack of consistency. Playing bass clarinet and clarinet are two different animals. While the fingers are mostly the same (keys vs. tone holes), often the embouchures are slightly different because of the mouthpiece size and angle, reed vibrational characteristics. If they don't spend enough time on each one, they will never gain a level of expertise on anything--jack of all trades, master of none.
When it comes to saxophones, in my opinion, make them play them all. The more time they spend on the low instruments, the better they will sound on the high ones.