After asking a few more questions, we knew this would be the child's first OTB tournament, and she would play in the unrated section. This makes our answer a little easier.
For anyone's first tournament, you don't need worry about equipment. Just go there and play. If both you and your opponent don't have chess sets, you (and TD) should always be able to borrow one set from other players. If there is no chess sets to borrow, for big tournaments like Chicago Class, there is always some chess vendor on site, you could buy one from them. And for unrated section, clock is usually not required. Therefore there is no need to worry about anything.
US is different from most other countries in the world, the organizers are not providing chess equipment, other than the scholastic tournaments. So players have to bring their own sets. If you want to buy a set, I suggest to buy it at wholesalechess.com. You can use the link on the right of this blog, which is embedded with my credential so you are supporting my blog. If you want to buy a clock, my suggestion is "DGT North American Chess Clock". But I suggest you search online first to compare prices. The prices of clocks are different from site to site, also the difference is not small.
But even if you don't have any equipment, you are welcomed by most TDs at most tournaments.