We just moved into our brand new home and our bathroom will get hot water in the sink but the tub won't get hot water at all... just a little tepid... all other faucets get hot enough to burn... what's going on

The volume to the tub is much, much greater than to the sink, and you may also have debris clogging the hot water pipe.
When you say new home, do you mean just built?

Shut the water off to the tub d remove the cartridge, Check and clean the inlet ports. Now turn the water back on for a few seconds to flush opt the supplies, reassemble and test. Better now? Good luck, Tom

Tom, they may not know how to do that, and if it's a brand new house, the builder should send someone out.

What kind of a tub are we talking about... a soaking tub, a jacuzzi tub, or a tub and shower unit? If this is a dedicated tub, such as a soaking tub or a jacuzzi tub then most likely you have a tempering valve located under the sink (or somewhere else nearby) and it needs to be adjusted to the proper temperature This should have been done by the plumber/contractor and tested by the plumbing inspector at final inspection.

If this is an older home, but is just new to you, then more likely is as Tom presented and you can try the repair yourself OR ask your home inspector why he missed that... should have been caught during home inspection!

Mark