YOU SAID:

I went to the park with my grandma and she peed her pants

INTO JAPANESE

私は祖母と一緒に公園に行き、祖母はズボンをおしっこをしました

BACK INTO ENGLISH

I went to the park with my grandmother and she peeed on my pants

INTO JAPANESE

祖母と一緒に公園に行ったら、祖母は私のズボンの上でおしっこをしました

BACK INTO ENGLISH

When I went to the park with my grandmother, she peeed on my pants.

INTO JAPANESE

祖母と公園に行ったとき、祖母は私のズボンの上でおしっこをしました。

BACK INTO ENGLISH

When I went to the park with my grandmother, she peeed on my pants.