Is it okay to ask a coworker/supervisor to give a talk? I'm a staff scientist in a medium-sized lab and I'm organizing a large event for high school students at my university, a large public research institution. I'd like to invite a professor or senior researcher to give a talk at the event. 

Is it generally considered okay to invite your close collaborators (e.g. PI, direct supervisors) to give a talk? 

I have no misconceptions about the importance of my event (it's not) but I don't want to violate some etiquette by asking someone in my lab to give a talk. What about other events, small conferences, etc.?
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.