zoom lens DOF, & Pentax over Minolta?

[Voyager]

All I use are primes, but my granddaughter has begun using a Minolta Elite AF in manual mode with 35-70 zoom. She tried my Nikon w/ 28mm lens, but was more familiar with a zoom, so rather than discourage her... However, I am totally confused about how to manage depth of field with this thing, since there are no f stops on the lens.

Also, I suspect that one of the Pentax ZX cameras might be a better overall zoom camera for her. What say on either or both accounts, please?