Christina Aguilera's Fiance Caught DWB ... Driving with Baby in His Lap

Christina Aguilera's fiancé gave their daughter her first driving lesson ... unfortunately she's only 18 months old.

Matt Rutler took the fam out for a Sunday drive recently in Brentwood, and while stopped on the side of the road little Summer Rain hopped in Daddy's lap. No big deal, until they drove off and she remained in that dangerous position.

Based on the pics, it appears Rutler only drove about 100 yards or so down the road before pulling into a parking spot, but it's still a clear violation and extremely risky -- remember there's an airbag just inches from Summer.

Rutler could've been hit with a $475 ticket for a first offense, but cops didn't witness it ... and luckily, no one was hurt.