The Washington State Patrol said troopers found three children locked in the trunk, after pulling over the speeding car on Interstate 405 near Bothell.
Spokesman Keith Leary said Anna Boyle, 28, and her fiance Aaron Johnson, 27, were booked into Snohomish County Jail Sunday on investigation of multiple charges, including drug possession and reckless child endangerment.
Boyle, who was driving the car, didn't tell troopers about the children in until the troopers heard thumping sounds from the trunk.
Leary said troopers opened the trunk and found Boyle's children - ages 5, 7, and 8 - and a small dog. They were taken into protective custody.
The spokesman said Boyle told police the children had crawled back there, but troopers noted that the back seat was filled and there were no child safety seats.