Taking to Instagram, the former Pretty Little Liars star shared an emotional post to congratulate the four-year-old.
"My baby graduates preschool today. My heart is breaking. Time moves so rapidly and the more I try to slow it down the faster it slips away. Here’s to all the mamas this week who are watching, from the sidelines, their babies move into the next stage. It’s not for the faint of heart ❤️ 📷 @alicia.alvey.photography," she wrote.
Tammin shares Phoenix with husband Sean McEwen.