The 3DS being download only will not be kid-friendly, so i doubt it. Anyway, the patent for the 3DS cartridge was leaked a few months ago, and it seems that it can store between 3 to 8GB of content.

I would stop buying handhelds

Being a portable console there are many benefits to it being DD,  And i would possiblly use my PSP more if I didnt have to carry around a bag with me and all the games.   But it sounds like the 3ds wont have to worry about this as it seems there is a cartridge already designed for it.  But I dont think the PSP2 will be DD either,  the failure of the PSPGo might make Sony wary of launching it as a DD console only.

I would be perfectly fine with a download only PSP2 and/or 3DS. Even now, I prefer having digital copies of my PSP games. I will only pick up a PSP game on UMD if it isn't available through PSN.

well the itouch is download only, and it's a huge sucess

Why would it not be kid-friendly?

Its not a huge success because of its games,  its an MP3 player first and foremost and thats what has made it successful

I would still buy one or the other, but it would hurt sales if either is download only. Also the lack of backwards compatibility would be a negative (assuming). 

sounds good, but it's all about the software.

While at the launch of the PSPGo Sony made many a fumble I think with the exception of pricing of the console itself they have done much to remedy the situation. Costs of 1st and 3rd party games are going down to the level of iphone and ipad apps anyway.  Many are still above that 14.99 baseline.  But we also forget that these games can and often do offer more content and gameplay than many of the smaller titles one tends to find on the app store (not all by any means). 

OT: It wouldn't bother me one bit.  I would welcome it. God forbid a portable be as portable as possible.