Crayola do a range called Beginnings especially for under 3's but if your supervising their superwashable ones are fine too (supervision re lids ) and they are actually washable unlike some of the cheap knockoff ones which if it comes off their clothes makes them worth a few more pennies IMO
I have picked up some felt pens that are really good for little ones. They have a flat base on the felt, so they can't push them in. Sort of like a felt stamper pen, but without a stamp, just a round base. I've bought two kinds, one at Sainsburys in the stationary section and one at ELC. Unfortunately, I can't find an online link to either product, but it might be worth a snoop around if either shop is close to you.