I honesty don't care if my toddler cousin thanks me or not

Your grandma is just trying to teach her to be polite. That's what grandparents that are like second parents do.

ITs okay to not care , but children have to be taught good manners, it’s Someone’s job

I agree with everyone else here. It's not for your benefit that she needs to thank you, she needs to learn how to get along in the world, and there are others who will care.

Your caring is irrelevant. A child has to be taught how to behave properly.

you speak like a bratty child