I have a 3.5 and just turned 5 y/o girls here during my ds's school time. I just pull them up to the table with us. They have a basket of activities (whiteboard and markers, sorting bears, Play-doh and a few cutters, stickers, etc). As they work I point out letters, shapes, have them count, etc. It's all very informal, but they're learning a lot. During times where I need quiet (reading and spelling) they watch Leap Frog or look at books. They love to listen in on storytime, then I read them a couple picture books (they each choose one).