if both parents want a child to engage in a certain extra curricular activity and that carries expense, than i don't think it's unreasonable to ask for them both to commit to paying for it - but i don't deem that "child support" as such - i deem it to be paying for extra curricular activities.

child support is to help, within each parents means, for the essentials of raising a child. extra curricular activities are nice, and yes, they can help developmentally - but there are many children both from broken homes and from stable two parent families that simply don't engage in extra curricular activities - whether it be due to time or financial constraints. they are not an essential part of life (and yes, i know some people will want to argue this - but they are NOT essential - they are a want, not a need)

if it is only one parent wanting for the child to engage in a certain activity, then i dont' think it should be expected for both to foot the bill. this is just my personal opinion. due to location, my DD will not be able to engage in many, if any, extra curricular activities. as her parent, i won't let that disadvantage her. we simply don't have access to anything without ridiculous travel and expense. she will have to grow up knowing that she can't do everything she wants because its' not viable. i don't think it's a bad thing for children to learn from a young age that there are limitations to what they can do, that money doesn't grow on trees yada yada yada.

my dad was unemployed for most of my childhood due to health reasons. we couldn't afford extra curriculars all the time - i don't think i turned out bad for not being able to do all the different activities my friends did!