Melissa Etheridge & Ex-GF -- We Hide Our HATRED for the Sake of the Kids

Melissa Etheridge reunited with her ex-GF Tammy Lynn Michaels for a mommy & mommy meal with their 5-year-old twins this week ... but afterward, Tammy let the world know they still DESPISE each other.

The gang met up for an overpriced lunch at Jerry's Deli in Woodland Hills, CA -- but when she got home, Tammy tweeted, "Sometimes you gotta plaster a sh*t-eating grin on your face for the sake of the kids.:-) "