To determine if the solution is innovative, consult the rubric.
Can they have an app built at their direction and have it count as "their" solution?
No. It is more important for the kids to do the work than to have a professional working solution. If they have app idea, do what is age appropriate for your team – poster boards, paper mock-ups, etc.
VA-DC Judge Advisor
My team would like to build an app as their innovative solution to this years project. It's nothing like sticking an iPhone in dirty water and having the speakers somehow displace contaminants to the bottom. It's more to help raise awareness and potentially provide support.
My question is: Can they have an app built at their direction and have it count as "their" solution?
The fallback, and maybe even main presentation for judges, would be a poster board sized phone to help simulate the app experience. They'd still describe their problem and how the app fit into their solution, they just wouldn't have an app.