Britney Spears Explores Berlin with Sons During Kevin Federline Child Support Battle

Britney Spears doesn't subscribe to the "More Money, More Problems" theory ... or so it seems 'cause she's having a helluva time with her 2 sons in the midst of a child support battle with Kevin Federline.

The singer and her 2 sons -- Sean and Jayden -- hit up a sightseeing tour Sunday at the Lustgarten in Germany. Awesome way to enjoy a break from her concert tour. They all seemed to enjoy the sights ... under the watchful eye of security. But back in the States, not everything was coming up roses.

TMZ broke the story ... Britney's legal team squared off Friday with Kevin, who has demanded way more in child support than the $20k per month she's been forking over. He wants $60k a month ... and she's willing to go around $10k a month higher, although she says she pays for everything and all he wants is to have her support his 4 other kids.