We can't believe how big Heidi Klum's youngest daughter has gotten! It seems like just the other day Lou was a baby...

The German supermodel was snapped taking her girls Leni and Lou to the Josephson Academy of Gymnastics in Culver City on Saturday afternoon, and she was joined by boyfriend/bodyguard Martin Kristen.

Even though she's long since quit her gig with Victoria's Secret, the mother of four still has her hands full at home and on the job. Project Runway's 11th season premieres this month, and Heidi just released a new line of activewear with New Balance, along with her own collection of children's clothes, bedding, furniture and accessories at Babies “R” Us.