Happy Birthday, Jayden James!!! It's time to celebrate, both your adulthood and the fact child support from Britney Spears will not end on this special day.

Sources with direct knowledge tell TMZ ... the original child support order in the Britney/Kevin Federline divorce provided that child support payments would cease on the 18th birthday of Jayden, the youngest son, OR when he graduates high school, WHICHEVER IS LATER. We've learned Jayden will graduate from high school in Hawaii in November -- it's an early graduation -- so the child support payments will continue until then. That means Britney will make payments in October and November, and then it all stops.

We're told the original child support order was $20,000 a month. But, in 2018, it went up to $40k a month, partly because Kevin pretty much had the kids all the time, and there were additional expenses as the kids got older.

Our sources tell us last month Britney and co. decided to cut child support in half, without telling Kevin's people. That triggered a legal skirmish, where Kevin's team argued if they ended up in court the judge would give Kevin way more than half, because he still has a number of fixed expenses. In the end, we're told they agreed to the $20k a month, but Britney also paid Kevin a significant amount of money to account for those fixed expenses.

As we said, the $20k a month will continue through November.

On a happier note ... we've learned the ice has thawed a bit in Britney's relationship with her kids. They are now occasionally speaking on the phone. As we reported, the only communication for a long time was via text, so at least they can hear each other's voices.