How can I keep my employees happy?

A few things to think about. Having been a founder and currently a CTO, I find most software people are actually not that hard to keep happy, but it is easy to piss them off or create problems unnecessarily.

1. Be transparent with them. The day will come you have to override a technical decision for a business reason, tell them the truth be direct, honest & transparent.

2. Provide them with feedback regularly. It should be honest feedback but recognize that even when things are broken it takes a lot of energy so show them that respect.

3. Highlight & publicly praise people for doing things above and beyond. Don't expect it, and even people that say they don't like it, appreciate it in the end. This is because a lot of times it isn't for them, it is so the rest of the team sees it and appreciates them and sees they can get the same.

4. Don't micromanage their tasks. Set goals and hold them accountable, but don't be the PHB (https://en.wikipedia.org/wiki/Pointy-haired_Boss).

5. Pay a good wage and provide solid benefits for people. You don't need to supply crazy benefits, give good pay with fair benefits.

There are a ton more things I could add, but these are common things I see people make lots of preventable mistakes and cause culture and team problems.

I would say that helping employees feel like they are making a difference with their work is important . It's very easy to feel like work is just a paycheck. Once people get to a point that work is a just a paycheck then the quality of work and happiness falls. Also help with work life balance. It's very common for new and young employees to give their all to new jobs they like but that leads to burn out which is bad for all involved. And eventually leads to bad job performance and having to fire them or they quit. As a CEO be very clear and constantly reinforce what your company culture is and what your goals as company are.

You drive the company and your employees are the force to get you where you need to go.

Also be careful with buying their loyalty via ever increasing benefits. This leads to a cycle of being disappointed with what you have and wanting more, no matter how much you get.

In my opinion, I would ask them first. Each company builds it's own culture and your team is small enough that you can customize perks for them.