Hi littles.
Yes, it does get scary when we have surgery and the body feel different than it should. There are many reasons why your knee may feel this way right now and it is OK. As your knee heals it will start feeling normal again. Sometime after something like knee surgery they give us medicine for the pain. That medicine can change how we feel. It can also make thinking hard and sometimes make the scared worse.
It's OK though. Take some deep breaths and see if there is anyway you can comfort or distract yourselves.