The Britney Spears meltdown express continues to gather momentum as it motors its way toward inevitable disaster. Recent headlines have included the spiraling starlet's grease- and poop-besmirched photo shoot with OK! magazine, her pitiable pole-dancing for a new video and her Sin City run-in with paparazzi.

Now, amid talk that newly minted ex-hubby Kevin Federline may be preparing to seek full custody of Sean Preston, 22 months, and Jayden James, 11 months, the tabloids are putting Brit's maternal instincts under the microscope.

"Help!" pleads the latest cover of Us Weekly over the down-in-the-mouth faces of Britney's boys. "Kevin battles for Sean and Jayden as Britney grows more dangerous."

Among the supposed parenting crimes Brit's committed: She's unwisely indulged the sweet tooth of Sean Preston, whose bumpy first year of life included a tumble from his high chair, a lap-set driving lesson and a close call with some Manhattan asphalt after Spears stumbled while toting him (both he -- and the drink she was carrying -- survived unscathed).

"He's having dental problems because Britney just shoves a bottle of juice in his mouth all the time to stop him from crying," a "family insider" tells the mag, which claims he's also allowed such treats as soda in his bottle, ice cream before bed, Doritos and -- Heimlich alert -- chewing gum.

Alas, instead of, say, altering her behavior for the future health of her son's enamel, Brit tried a much quicker fix, says Us. In April, the sense-challenged Spears allegedly asked a Los Angeles dentist to whiten SPF's baby teeth. The dentist said no way.

Britney's balmy behavior was also in evidence during a recent house-hunting expedition, when she seemingly made herself way too comfortable while checking out a Pacific Palisades pad listed for $6.5 million.

"She ate tacos on the bedroom floor!" maintains a mole. "She got crumbs and grease everywhere. She let her babies and her dog roam the floor. She left the bed a mess -- I don't know what's wrong with her."

One source chalks up her id-driven attitude to immaturity.

"She is just so used to being the kid herself that it's babies having babies," the snitch posits to the mag. "Believe me, I'm no Kevin fan, but thank God those kids have him. He is the only normal thing in their lives."

A head-shrinker agrees, diagnosing to Us, "She strikes me as someone going through a delayed adolescence."

And it's the same sad story on the front of this week's Life & Style, which features the siblings' chubby-cheeked mugs over the slightly more beseeching headline, "Britney's Babies: Help Us!"

The magazine charges Spears with making Sean Preston a party to her nicotine fits, which is quite a change from the days when she wouldn't allow K-Fed to puff in the house.

"She smokes like a chimney in front of them," a mole tells the mag, adding that when Brit is craving a cigarette, "She'll actually turn to Sean and say, 'Baby, where are Mama's lollipops?' Sean runs, gets her cigarettes and brings them back to her."

(Hey, what could possibly go wrong with a kid associating candy with cancer sticks? Oh, wait ...)

"It's tragic what these boys are exposed to on a daily basis," the insider continues. "Kids thrive on routine and the feeling of safety, and Sean and Jayden have neither. Their lives are subject to Brit's whims."

Those whims are also said to include her "short attention span," especially when the boys are cranky, and her continued fury with mom Lynne, whom she's reportedly warned to keep away from the kids.

The source alleges that when Sean asks after his grandma, Spears "tells the boys, 'she's a bad lady.' It's so inappropriate to drag the boys into her family feud when they can't understand what's going on."

But not everyone thinks Brit's nurturing technique is a nightmare. Her former assistant, Alli Sims, is speaking out in her defense.

"She's a wonderful mom and she is so hands-on," the ex-minion-turned-would-be pop star gushes to People. "Britney loves those babies to death. And they adore her."

Next: Doherty Disses, Misses Moss