Rats are known to be relatively quiet animals, but that doesn't mean they don't make any noise at all. Baby rats, in particular, tend to squeak quite a bit. This is perfectly normal behavior and nothing to be concerned about.
There are a few reasons why baby rats squeak more than their adult counterparts. First of all, they're simply much smaller, so their vocalizations tend to be higher-pitched and more easily heard. Secondly, baby rats haven't yet learned how to control their vocalizations as well as adults have. They're still experimenting with making different sounds and trying to figure out what gets them the results they want (attention from mom, for instance).
Eventually, baby rats will grow out of this phase and start to squeak less often. In the meantime, it's just part of having a young rat around the house!