Hi Andre M,
I'll defer to Eric from APVM here.
I have never been given an IDO to try out, so I have no experience with it.
I still maintain that good instruction combined with a tether system for instructor
control will get newbies up and sailing sooner than something like the IDO.
Also, the IDO can result in the board (with or without the sailor on board) sailing off
on it's own, so some sort of rescue craft is pretty much mandatory if you are using the IDO.
See Ellen's comments from her experience with the IDO last year.
Also, are you considering that the GO 155 or 175 does not have a centerboard or provision for a center fin. The older GO boards had optional side fins that provided "rig steering" for absolute beginners, but the current catalogue does not suggest that the new GO boards have this option.
This being the case, for absolute beginners, the Rio or Start, with a centerboard, might be better for the beginners in your family.
Especially for the really young lightweight sailors, foot steering is quite difficult and they are pushed downwind because there is no center fin to provide lateral directionality and also rig steering.
Hope this helps,