Answer: They're, There, Two, To
Explanation: "They're" is short for "they are" so think of it as "I'm not sure if they are planning on going to the park with us."
"There" can be used to determine where something is. "There is a park close to my house."
"Two" is a number. "Two" cookies, or "two" cats.
"To" is a little difficult to explain, because it has different definitions. You can search up "To definition" for a better explanation. Hope this helped!