My son wants laptop/pc for christmas and I don't want to spend more then $400 on a computer.$500 absolute tops. He plays some game call roblox(i think spelling is correct), minecraft, and something called dayZ. All i know it is very slow and i hear him yell at the computer because it lags.
What is the best cheapest gaming computer/laptop?
For a gaming PC/laptop, make sure the rig has 8gb RAM (at least 4) and a decent graphics card:
Radeon HD 7770
Radeon R7 260X
Geforce GTX 660
Ask your retailer about a cheap PC that has one of these GPU's or something equivalent.
EDIT: Don't listen to the previous post (The brand is not the key thing to be looking at!)
The games you have mentioned usually need certain minimum requirements for playing them well. So it is better to see a laptop which is fulfilling the minimum requirements to play these games.
For your ease I have one laptop in my mind to suggest. It is from Dell with the price of under $500. It is good laptop for the low to medium settings games. You can see here a review as well which is really helpful in making decision:
http://whatlaptop.org/good-gaming-laptop-500
The review shows that it has 3rd generation i5 processor, good memory of 4GB and decent storage of 500GB. Also it has good graphics and battery life. So overall it a good laptop for gaming under the price $500.