"Ok, make sure you look left and right." Each boy holds on tightly to their father's hand as he carefully walks them across the road. As he lets go, they run into the park.
"This is the pirate play park, we go here every day after I get back from training," says Mark as he catches them up. "The one thing that's been difficult about being a Dad and only having one leg is not being able to chase after them. That's why I've been so careful to teach them how to cross the road with me."
Mark hasn't been always been able to be around his family as much as he wanted to. As a former Lance Sergeant in the Queen's Company of the Grenadier Guards, he was often on tour or away for training.
Mark joined the Guards straight from school, and met Natalie, his future wife, the day before he went back to Bosnia.
After tours in Bosnia and Iraq, he experienced one of the toughest tours of Afghanistan in 2009. Three of his colleagues were killed by a rogue member of the Afghan National Police and another colleague was killed by an IED.
This all changed when he was injured in 2011 during a live-firing exercise in Canada whilst training for his second tour in Afghanistan.
"I was standing behind a wall when the shots came through and hit my upper thigh. It took three days before the decision was made to remove my leg."
"With all the time away, Natalie hardly saw me. I met her the day before I went back to Bosnia and our relationship had always been long distance. After I got injured we had to adjust to me being there all the time."
After two years of treatment and physio at Headley Court Mark was in a difficult situation. He was no longer receiving support from the military and was about to adjust to a new life as a civilian.
"I tried a few jobs but I knew I was unhappy. I wanted to be a role model to my kids. I couldn't be that as a solider anymore."
Then Mark became interested in competitive bodybuilding. During his time in hospital he lost weight, and he wanted to put it back on in the right way. He became involved in the sport, and shortly after he started training he was competing and winning awards.
"I have to practice my posing at home and the boys now join in. After my first show Natalie told me it was the happiest she had seen me since my injury."
"I'd love to do Dad things like go to the pool with them, but we do get to do other things together like bowling and the cinema. When Natalie's away we have a normal boys only weekend!"
Mark says that the injury has made his boys more caring. "I still have to have operations on my groin. They always know when that is and make sure they don't sit on 'Daddy's poorly leg'."
Reflecting on what it means to be a dad, Mark says, "I feel fortunate that I'm able to watch my children grow up because, with the amount of times they resuscitated me, I shouldn't be here. The moments when they come into our bedroom in the morning and jump into bed with us, they're bonuses really."
All photos copyright to The Royal British Legion / Alison Baskerville.