You can try to bribe them, but really I think therapy only works if the person wants it to. If they still live at home and are dependent on you then you could say it is a condition of living with you, but you'd have to be willing to follow through and kick them out if they don't go to therapy, not sure how that would play out long term.

Really its no wonder they would resist you on this. I think just continuing to bring it up, unless there is something majorly wrong with your kids, will cause a major rift in your relationship if there's not one already. Essentially you're telling them there is something wrong with them they need to fix to be acceptable to you... normally people go into therapy when there is something wrong with them that they see and want to fix to be able to live better.