(Taking care to work with in the bounds of your 9-5 job/contract, I wouldn't bring it up with your boss/co-workers but make sure you're not breaking rules in case it would every come up). Get an EIN number and work as a 1099 contract dev with clients, LLC if you're bringing in lots of hours over the year.
I'll mention the app store seems like minimal return on investment if any for most iOS devs so I wouldn't expect to make money off that. If you enjoy it just want to roll the dice without any expectations more as a hobby but sounds like you want to bring in revenue.
Beyond that investing 12 hours in building an audience for a product (ebook/training/info products) might be worthwhile.
Give this podcast a listen . . . http://www.startupsfortherestofus.com/
Microconf talks https://vimeo.com/user12790628/videos
Building a product or SaaS app times time/is hard but having recurring revenue down the road is a huge payoff . . .
Congrats to letting your wife pursue her dream.
Good luck.
Then, if you could do that in your 12 hours of free time, you'd have turned 12 hours of your time into all the money you could ever want, and used it to do whatever it is you want to do.
1) search angellist jobs section for companies looking to hire their first iOS dev, or to have an MVP of their app made 2) contact all of them (this is a numbers game) by pitching yourself and how you have the exact skill set they need, include a portfolio, and mention your terms up front (hours per week/desired rate/no I don't work for sweat equity)
Even people looking to hire full timers are probably desperate enough to consider taking on a part timer, even as a temporary stop gap.
I prefer angellist jobs section to traditional freelance sites because you can avoid race to bottom bidding and avoid much of the competition. There are a lot of open minded people looking for high quality workers out there and are thus willing to pay a better hourly rate, but they avoid most freelancing sites and focus on full time because they don't think there is an alternative- be that alternative to them
Making an app is great when it works, but it's not guaranteed to earn anything. It can take a while to find the right product. Even then, you're better off having a diversified portfolio of products, it's unlikely that one product alone will replace a full-time income, and they'll have individual dips & peaks in sales / subscriptions (several products helps make your income more steady).
Contributing to open source guarantees you won't earn anything upfront. Freelancing / consulting is far more immediate income, you either have a paying client or you don't. It's possible that some of your prospective clients won't even know what open source is, let alone be able to judge the quality of your code.
Of course, there are lots of ways to earn income that don't involve code. Getting casual work at Starbucks is an option. Being a day trader on stock markets might be another. Not everything has to be solved with code.
But the OP asked specifically about earning money to replace a full time income, and consulting is a faster route to money. Product businesses take time to build an audience (you do have an email list of prospective customers, right?) and you're looking for thousands of customers. Whereas with consulting, you only need to find a handful of clients. Charge your client $100/hour (really you should charge more, or charge weekly) and you'll be so far ahead of most App developers. Which is easy, since most apps earn no money and many get no traction at all.
You could always try both. Split half your time into consulting, the other half into doing an app. See which works better and gets more traction. Hopefully the consulting half becomes unnecessary as the apps take off and become a hit.
As a senior developer, you could try consulting on the side. Do you network? If you're good at what you do, there's probably someone you know or someone you know who knows someone who might be able to use your expertise. It never hurts to ask around.
That's all I can think of. They're probably not new ideas for you though. Hey, you can always contribute to open source software. It won't make you any money, but you might get your name out there and help a cool project out in the process. You might even learn some new things to put to good use in your career.