Khloe Kardashian, Tristan Thompson Celebrate Daughter True Starting First Grade

Time is flying for Khloe Kardashian and Tristan Thompson ... because she's getting emotional over their daughter's educational milestone.

Khloe and Tristan gave True a grand sendoff to first grade Tuesday, with a fancy photo shoot and gushing social media posts ... and Khloe offered a glimpse into how she's feeling as a mother.

KK says ... "I'm not ok but I'll pull it together by pick up time."

She added ... "First Grade!!! True I am so proud of you!! I know you're going to have the most incredible year ahead of you! I couldn't be happier for you my angel."

The proud parents, who are separated but co-parenting, each shared a series of photos with True in her school uniform ... cheesing for cameras in front of a background filled with balloons, giant crayons and a "First Day of First Grade" sign.

Khloe and Tristan took turns squatting down and hugging True ... and her little brother Tatum even made some photo appearances ... ditto for her pet cat.

Tristan got emotional too, mentioning his late mother in his post ... "I know Gigi's looking down on you and she is sooo proud of you and how amazing you are and how you're the best big sister."