If you know where your parents are going beforehand, such as a planned trip to the park, you can arrange a meeting with this person you wanna meet. Get as much details without being suspicious, such as time, place, etc., etc.
1. tell your parents u wanna go to the part
2. bump into them 'on accident'
3. "OML!!! IS THAT U, [so and so]?? IT'S ME, [ur name]!! I HAVENT SEEN U SINCE 5TH YEAR/4TH GRADE"