Birthday Party help!

My daughter's birthday is the 15th of March, my son's is the 19th (also my hubby's and niece's is the 11th!) and this year we can actually afford to throw them a big party like we did 2 years ago (ds's 1st b-day and dd's 3rd). And since right in the middle of their birthdays is a Saturday we thought we'd do it then (St.Patrick's day lol). The problem arises when I ask them what they want for their party "theme"! My dd REALLY REALLY wants Unicorns and I found a SUPER cute party set at BirthdayExpress.com that she is absolutely in love with. But my ds keeps changing his mind about what he wants, of course he's only 2 so I don't expect him to make up his mind that easily but I need something that SORT of goes with the Unicorn theme, or at least doesn't clash HORRIBLY with it, kwim? His favorites right now are Robots, Trucks (construction trucks cause they're yellow :P), Hot Wheels, or Spongebob (@@ i hate spongebob but he loves it cause its yellow, lol my son LOVES yellow). We were TRYING to get him to choose a dragon one so it would be Unicorns and Dragons or to go with something that suits them both like Max &Ruby but neither one would go for that :P SO, my big question is: Should i just let them both pick what they want no matter how weird it goes together or should we try and compromise on a mutual theme? And if separate ones how would you decorate for em, like mix it all together or try and divide the room/party into each different theme? (not sure where we're having it at yet)
Oh and the last time we had a big Pirate theme that they both LOVED as did all our guests :)

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