I'm sure this was asked some time ago but I cant find the thread so I'll ask again!
I'm off on holiday to escape the cold / royal wedding and I've just ordered a portable Genesis http://www.blazeeurope.com/sega/sega-ultimate/prod_133.html (got mine for £31 delivered) to keep my 5 year old entertained on flights & car journeys when all else fails. It has an SD card slot so I can transfer my Genesis ROMs to it but I don't want a card with hundreds of ROMs on it just a few games he might like.
The only Genesis game he has played before is Sonic and to date we haven't let him spend much time playing games just very simple web based stuff from kids TV show sites and a bit of Pac Man and Galaga.