i have to totally disagree with this common perception of catalan in ibiza.
the comparison that british people make between it and welsh is absolutely ridiculous. not only does catalan have a literate population comparable to that of hebrew, greek or czech, and more than languages like swedish or danish. but....
a) it is the predominant native language of the island. imagine going to live in holland and not learning dutch, just because everyone speaks english.
b) it is a historic european language that should be preserved and protected as much as possible. otherwise, in 100 years, it will have disappeared almost.
c) it is the language of governance (i can't think of any better way of being disenfranchised than not knowing catalan)
d) all jobs for the local councils require catalan, so one of the best sources of annual, well paid, long term for your kids could be forgotten about.
e) people say only the older generation speak catalan. that is utter rubbish, my normal day consists of speaking in catalan to the people i meet and places i go more than spanish...(school, supermarket, gym, coffee in ibiza town, etc, etc).
finally, if the above doesn't sway you of the importance of catalan, then knowing it will make learning french, or any other romance language, much easier for your children.