Ooh gosh, new posts, thank you all so much for the suggestions 
I have sort of parked this for now. I am still undecided, though I think I will go for wool of some sort or another natural fibre.
I am tempted by acrylic for all those reasons Zing, but I think wool is perhaps a bit more special.
I like bright colours normally. But I am thinking of using cream/white, against all advice. Partly because most of my family are at least nominally Catholic, so then they could use them as christening shawls if they wanted to.
Also it's hard to know what will be in fashion in future or what my cousins' tastes will be. I imagine some of my cousins would have perfectly coordinated themed nurseries, but hopefully cream or white would go with almost any colour scheme.
To be honest, I am a bit sad about starting the project as it feels like an acknowledgement that I am dying (though my chemo seems to be working well, so I should stress I am not in any imminent danger!). Also I am worried that I will not finish them all. Obviously the sooner I start the more likely I am to finish them, but I'm not in the most rational frame of mind.
I also live with my parents and I am a bit worried this is going to upset them. I think I will have to knit/crochet in secret 
I think I am going to have to take a trip to the lovely wool shop a few towns away and feel lots of different wools. Hopefully something will jump out at me when I see them in real life!
Kittens have you knitted anything yet? 