Britney Spears Hangs Out with Paul Soliz on Valentine's Day

Britney Spears and Paul Soliz seems to be pretty serious ... 'cause they're hanging out on the most loved-up day of the year -- with Soliz's kids in tow.

The singer-songwriter met up with her on-again, off-again beau on Valentine's Day ... driving around L.A. and enjoying a low-key date night.

No Craig's or Giorgio Baldi for this happy duo ... the two grabbed fast food at Jack-in-the-Box instead with his kiddos in the backseat.

They went through the drive-thru -- a shocking moment for whatever teenager was working the window, we gottta imagine -- and, after leaving, Paul stopped and grabbed flowers for his boo.

The two were spotted laughing it up in the car together ... so, looks like a very successful holiday for these lovebirds -- good news since their relationship hasn't always been easy-breezy.

As you know ... the two had a blow-up fight in May of last year which nearly landed Brit in the hospital -- and, in July, Spears said she was done with Soliz completely.

Not long after they were hanging out again ... with Britney going on multiple excursions with Paul and his kids -- including one at an indoor playground earlier this month.

BTW ... unclear how Britney ended the night with Paul -- but, she did post a video to Instagram that implies the couple may have had an eventful evening.