Should I leave for a 3 week trip

I may have an opportunity to go to another country for 3 weeks to do some volunteering but I am worried about leaving my 6 year old and 3 year old.

This seems like a once in a lifetime opportunity and the kids would be with their dad and their nanna would be there to help but I can't help but feel guilty and worried about leaving them for so long.

Would you go or turn down the opportunity?

Edit: I have now been told it is going to be for 2 weeks if things go ahead so I think I will go. Thank you.