Are there any exercises for people who have flat feet?

No need: Yes, but why? Toe raises make posterior tibial tendon work and restores the arch when toe walking. But flat feet are a normal variation that even world class athletes have, and thereby don't require treatment. An arch that collapses and is painful is different and should have a supportive insole to take the stress off of the failing pt tendon.

Heel cord stretching: Many people with flat feet have tight heel cords. The tight heel cord may make the flat foot worse and create a vicious cycle. Ankle strengthening may help some, focus on the posterior tibial tendon. None of these things will change the flat foot an create an arch, but they may help with pain and prevent it from getting worse.