Dells are nice because they give fine handholding, return priviledges (no shipping to pay), and will steer you to a good box. Athlons are OK, more bang for the buck.
In your shoes I would go with a fairly "slow" processer and a hard drive more like 40 gig. Also I would see my favorite used computer shop. I could go out right this second and buy a Pentium III with multimedia, CD, and 20-40 gig drive for under $200 (with likely more than 5 years of life left), or a good 17" Sony tube monitor for $80. Personally I'd load them with W2K.
Apples are very expensive for the performance provided. If you want a totally secure OS Linux or even OpenBSD.
We bought a good used, refurbished computer for our daughter from a local computer repair shop three years ago. It was a great deal and we've been very happy with it.
She wants her kids to use them, not be frustrated by them. Best bet is a Windows OS. Yes...Windows can be properly secured as well, but this is for kids.