Hi there, we have some service items that are divided in 3 categories (ex: session for childre, for adult, for senior) and these services could be sold by hour, by unit and by flat-rate package... How would you handle them please?

you can set it up as different price levels. assuming you have different rate by hour, by unit

That would be an idea indeed

You should also checkout Matrix Items. It might save you a lot of headache.

That was also an idea I've got