You can focus on why you paint:
Oh, I mainly paint at lunch time as a change from my work; I don't really do this as a major hobby.
Your coworker probably doesn't plan to bring his son to the office, so this should deflect the request on logistical grounds. But if not, here's an approach I've used:
I'm sorry, but I'm not good at portraits. Learning (type of painting you do) keeps me plenty busy already.
It's possible that the person doesn't intend to ask for a favor and wants to pay you. Doing business with coworkers can be a little risky (if anything goes wrong on one front it affects the other), and if you aren't already in business then having the money conversation can be awkward, especially when you're talking about a piece rather than an amount of time. (I'm a musician, so when asked if I could play at parties I could at least talk in terms of what I charged per hour. That's harder for a painting.) If this is your situation, you can say that you prefer not to work for coworkers.
Finally, I have a suggestion that goes beyond what you asked about. You're doing these paintings at the workplace during your off time, and you're intentionally doing them in public for social reasons. There are a couple ways that you can involve other people so that they'll feel like you (at least partially) painted for them, without taking on commissions that you don't want to do.
One approach is: if somebody admires a piece you're working on, offer to let him take it to hang in his cubicle. By specifying in his cubicle you're implicitly saying "I'd like this to stay here at work", but you're still sharing your work. If you don't care about where it ends up -- if you're painting to paint rather than to have paintings, then you could just offer to give it to him. (I'm assuming here paintings that are faster works, not something you work on every day for weeks before finishing it.)
The other approach is to involve people in the decisions you're making as you paint. If people are around and seem interested, you could say "hmm, should I brighten this part up?" or "I was thinking of putting a tree here" or "I'm not sure which color to use for this woman's sari" -- and then ask "what do you think?". Even if you don't need advice, inviting input and acting on it can build that sense of connection that led you to paint in public in the first place.