I am an agnostic , so being undecided myself , I won't care too much . I'll cover the cases one by one .
If my child joins Islam or Christainity , I will plant the seeds of doubt by leaving around the house materials which call the core beliefs of these religions in doubt , and I will recommend my child to be cautious .
If he joins Islam , I will tell him to be very sure before he makes any commitment , because the Islamic penalty for apostasy is death , and point him to other materials , such as the Hadith , which , if read in the original , are capable of shaking the faith of any liberal . I will then also point him to the records of Muslim rulers who destroyed Hindu India , and the language which they used . I will also point out to him the condition of women under Islamic law , and also the treatment of infidels , as also to how Muslim clerics behave even today .
If he joins Christainity , I will join him in his enthusiasm , then we will read the Old Testament together , and I will try to give justifications for the sort of bad behaviour found there done by God and God's prophets , and make it clear that I am making these excuses out of respect for my son's feelings . That should make him leave Christianity fast enough . If that does not work , then we will read the New Testament , and I will gentle raise questions about inconsistencies in the Gospels .
As Judaism does not accept converts , I am not worried on that front .
If he joins Buddhism , I will not object , but will tell him to be wary of the later interpretations , which are all negativist in nature . I will point out some of the more ridiculous implications of the later Buddhist interpretations .
If he joins Jainism , I will say/do the same thing .
And finally , if he decides to become an orthodox Hindu , I will tell him that being orthodox anything is bad for man's progress , and that he should be open to new ideas . If he becomes non-orthodox Hindu , I won't object at all ( as that is what I essentially am - a non-orthodox Hindu agnostic ) .
If he becomes an atheist , I will ask tell him that atheism is as untenable as theism , because unless you have yourself experience of the divine , you cannot decide either way .
I know that what I am doing can be called insidious ( specially the subtle planting of doubts part ) , but I consider it to be the "softer option" , because openly encouraging or discouraging beliefs never works . Why use a sword when a needle will suffice ?
on them, I'm sure that'd persuade him/her to come back to the fold. 
