Britney Spears is very much back on with her on-and-off BF Paul Soliz ... because they're enjoying family outings together ... with some indoor fun.

TMZ obtained this photo of Britney in a black Mercedes Benz G-Wagon outside Lily Pad Playground in the San Fernando Valley ... and folks who where there tell us she was rolling with Paul and two of his children.

We're told Britney, Paul and two kiddos rolled up to the indoor playground Tuesday ... with Paul driving and Britney riding shotgun.

Our sources say it's the same car Britney bought for Paul about a month into their relationship.

Britney's appearance at an indoor playground in the SFV came as a shock to other parents who were watching their little ones have a good time at Lily Pad Playground ... but we're told everyone pretty much let Britney be and didn't pester her.

There were some Britney interactions, though ... we're told she was overheard telling people inside, "These aren't my kids."

When it was time to go, we're told Britney paid the bill.

Britney and Paul have had a rocky road in public -- including an epic blow-up at the Chateau Marmont in May that almost landed Britney in the hospital -- but Tuesday's outing makes it clear they are back in the saddle.

This isn't the first time Britney's spent time with Paul's kids either ... she was photographed with one of his daughters back in September 2023 and last August she took 3 of his kids to a Sky Zone in Thousand Oaks, CA.