But back in March, his mom noticed he wasn't acting like himself.
"He started to complain of leg pain. I took him to his doctor. His doctor just said it's growing pains," she said.
Sarah knew something was wrong.
"A week went by, same thing. I took him back... found it to be normal, sent us home."
A month later, things took a turn for the worse. That's when they found their way to Valley Children's.
"The doctor came in with the nurse, and I noticed the nurse stayed by Maddox' side. And he just kind of stared at me, and he said, 'What we think he has is Neuroblastoma.'" Sarah said.
"Neuroblastoma is one of the most common types of solid tumors in children. It tends to be very aggressive in children that are older than four years of age," said Dr. Karen Fernandez.
Sarah remembers that diagnosis vividly. "It was the hardest thing to hear. I never thought I'd hear that my kid has cancer. I just asked him, 'Am I going to lose him? Is he going to die?"
Conditions like Maddox's are immensely complicated. It requires a lot of pediatric specialists working together very closely.
Dr. Fernandez says that's why Valley children's is so crucial.
"Valley Children's is the large pediatric hospital in the Valley. Having access to a medical center closer to home and being able to receive adequate treatment within 30 to 60 minutes, it's very important to minimize other complications," said Dr. Fernandez.
It hasn't been an easy road for Maddox.
But his mom is so, so proud of him.
"From the very beginning, Maddox has maintained this faith that he's going to get through this. He's the one who keeps me going. He's the one who's like 'I'm good.' He's not fazed by his chemo, like it's not even happening," Sarah said.
His doctors have taken notice, too.
"He's a very friendly, very happy kid. That it's almost like nothing has happened to him before."
Now that Maddox is receiving the best care available, he can focus on two things: Getting better and knocking out that next Lego set.
You can help support the hospital that's changing lives. Click here to learning more about the Futures Worth Fighting For Telethon benefiting Valley Children's Hospital.