The heavier and thicker you can make a welding table the better it will perform. Heat has a tendency to warp and bow the top so You want to use as thick a piece of plate as you can afford. The legs should also be of good quality square tube with thick walls and copious cross and under bracing. That way when you are persuading something to line back up with you 8Lb persuader you wont had divots and issues. I also tend to weld jig stops and alignment pieces right to the top and grind them off when I complete the project. I also use a reciever hitch type system for vises, chop saws, Bending bucks and grinders. They are easilly interchangeable and do not encroach on your work space when you really need all you can get. My table top is 4X6x.25 and has 2 inch square x3/16th thick wall legs and 1 inch square lower shelf suppots and cross braces. I had to heat the top to remove a bow but it is tacked down and solid as a rock now. The casters are on one end only and I use a hand truck to lift and drive it around the shop. I dont move it very often though. The shelf is made of expanded metal to allow dust, slag and schmutz to fall through without piling up and making a bigger mess. It's heavy, I would estimate 450-500 pounds and I do move it occasionally when using my 10Lb sledge. Once my top becomes damaged I will grind weld and recover it with a 1/2 plate after I get the bows and divots beaten back out so it is as close to flat as I can make it. I would strongly recomend not trying to go light or cheap on a good welding table, it should last your Grandkids there entire lifetime with no problems.
Bob