Britney and Kevin Federline have joint custody of 13-year-old Sean and 12-year-old Jayden. The arrangement has been that she gets them 3 days, he gets them 3 days, she gets them 3 days, he gets them 3 days, and so on.

Britney has had less than the agreed-upon 50/50 custody for several months leading up to her stay at the mental health facility. While she was doing poorly, the kids would visit, but Kevin had the kids most of the time.

So, Britney begins her 3 days with the kids today, but there's still significant concern about her mental state. Our sources say the 30-day stay at a mental health facility -- which ended Wednesday -- put her in a better place, but there are still issues with her judgment, and it goes beyond medication.

As we reported, doctors continue to struggle, trying to find the right combination of meds to stabilize Britney. The meds she had been taking lost their efficacy, and creating a new cocktail is fraught with problems. Britney had stopped taking the new meds earlier this year when they were doing more harm than good.

We're told progress is being made, but there's another big unresolved factor. The meds will stabilize her, but there are more fundamental mental health issues that go to judgment and perspective, and that requires intensive therapy ... something she has been and will continue to receive.

Short story ... she's doing better, but she's not right, at least not yet. It's something her dad, Jamie, has been dealing with for nearly a dozen years as her conservator, trying to give her the space she needs to live her life, and at the same time steer her away from disaster.