+ I.e. full height 2x2, 2x3, 2x4
+ I.e. full height 2x2, 2x3, 2x4
https://www.toysrus.com/product?productId=45199456&source=CA...
This looks pretty basic.
https://shop.lego.com/en-CA/Pick-a-Brick
There's a search filter.
You can filter by colour and by name, for ex: 2x4
Sure you can. If you just want non-themed sets heavy in classic pieces, they are sold as Lego “Creative Box” or “Creative Set” in various sizes [0]; if you want specific pieces in the common sizes, you can go into a Lego Store and handpick want you want from bins in the wall and pay by the bag.
[0] e.g., https://shop.lego.com/en-US/LEGO-Large-Creative-Brick-Box-10...
Maybe parent poster could buy a couple of kg of Lego from Ebay, then sort and wash it. The wheels, doors, etc can be donated to local charities / goodwill etc.