Is it okay to ask a coworker/supervisor to give a talk?
Yes, I think it's perfectly acceptable that you invite colleagues to give a talk at an event or conference.

There are many advantages for them if they give such a talk, for example:


disseminating their work 
practising speaking to a non-expert audience
networking
free coffee/ lunch/ biscuits


If they don't want to do it, they'll decline your invitation and you can ask someone else-- a good choice might be PhD students who will actively want or need to practice outreach and public engagement. 

Anyway, whoever you decide to invite, I don't think you've got anything to lose by asking.