If I have a pet with hidden traits but I dont test it (I know it has hidden traits from the sandboxes), can they still pass on their hidden traits to offspring? Is it also the same with carried traits?

yes- them not being genetested just means you can't see what the hidden (/carried) traits are, not that the genetics aren't there.

Okay, thanks!

This is a good possibility, it depend of the trait(s) the pet they were breeded with had. The letter next to the trait name also indicate it's dominance over others traits, a ressessive trait cannot appear from a trait that his higher then the visible one in alphabetical order. You can increass the likeliness of a ressessive to appear by breeding the pet that have it to another pet that have the gene or attempt to sandwich said ressessive between the visible trait it come with and a mate that have a trait comming after it in the alphabetical order. (exemple : AB+CC)

However carry traits have roughly 50% chance to pass down to the next generation and, again, depending on the trait the mate have you may lose the ressessive on the offspring, especialy if the trait of said mate is dominant over the hiden from your pet. ^-^