I have a wooden set from Ikea that can seat four. The table is VERY sturdy- the kids tend to jump off it and it's still quite stable. The chairs are kind of small, though, as my 5 YO is outgrowing them and they fall backward if the child stands on them. Since my children are rough on furniture, the screws have come out and they have had to be fixed a number of times.
I also have a telescoping folding table. It seats 6. It's just like the 'adult' rectangular folding table you might have at church or work but the legs telescope so it can grow with your children. Then you can buy chairs like they have at school and daycare. My table is compressed wood/laminate with metal frame. Here's one, although it is longer then mine
http://tinyurl.com/2zt7sf
This one at Costco is very pretty
http://tinyurl.com/3ykksr
And at Sams, here's a lot of different tables and chairs, both wooden and metal/plastic
http://tinyurl.com/33o8wt