I recently had my entire masterbath shower redone. It seems like ever since, my downstairs kitchen sick and upstairs kids' shower have no hot water now. What could have happened to cause this? We replaced the water heater thinking that was the issue, but no that issue still remains :(

Thanks,

A valve is still closed, or the plumber lost track of what was going on?

Also, when I turn on the faucet in the downstairs bathroom (which is super hot) the kitchen water goes from luke to super cold?

Start with removing all aerators and shower heads, tell us if anything changes

The shower has not yet been installed all the way in the master bath, they just finished all the pipe -work, so I have no shower head to change out in that bathroom. The shower head in the kids bath is fairly new. This isn't just a shower issue though... the kitchen sink is acting up the same way? Do you think that maybe the new piping for the master shower was done incorrectly and is causing all this?

What may be happening is when you did the work you disturbed some minerals built up in 56the pipe walls.and now they're blocking the hot water inlet ports on the cartridge.
First do what Bob suggested . If that doesn't help shut the water off to a affected faucet and open it up to remove the cartridge. Check and clear the inlet ports. Now turn the water back on for a moment to flush out the supplies. If you have a strong stream reassemble and test. Do this with all affected faucets. Good luck, Tom

I wonder if The unfinished shower has a cap at shower head, and with faucet on, it allows "backfeeding",
And a valve is off?

Thank you. We'll try some of your suggestions tonight.