Let me list out the pros and cons so perhaps you can decide.

Simplified:
Pros - used by more people as it is the official written language in mainland China (Taiwan uses traditional Chinese exclusively, and I believe Hong Kong still uses traditional Chinese for formal writing)
Cons - many words have lost their true meaning during the simplifying process (words are not only simplified to fewer strokes, but different words are also simplified to the same word)

Traditional:
Pros - retains more original meaning of the words
Cons - many official translations are only in simplified Chinese (for example, public places with Chinese signs, manuals, airplanes)

Personally, I grew up learning traditional Chinese, and it wasn't hard to learn simplified Chinese - you start to see the patterns of how words were simplified.  I didn't have re-learn the language, but instead was able to pick up naturally after spending a month in China, or many times you can just guess.  However, as your babies grow up, they will most likely only be able to learn simplified Chinese from school, so it may be easier to just start out with simplified Chinese.  Since this is their 4th language, I would think that simplified Chinese is sufficient (more useful too).

It's definitely a personal choice.  We chose traditional Chinese because our families are from Taiwan, but we hope our kids will be able to read both in the future.  Hope this is helpful.