If everyone think about kidnapping etc, then they would not even allow anyone to hold their child. I understand your point but sometimes people would come into your house, be nice to you and win your trust but they are really evil (look at Onir). They can still kidnap your child if you know them long or just a few days. It all depends on the individual and off course you have to put your trust in God. You can never really trust someone completely, everyone is human.
In archu's case, she trusted her sister, there was no way in her dreams that she would have seen or imagine that varsha, her own sister would have done such a deed.
I deliberately stayed away from the cradle thread but I would say that if Purvi does not see the need for a cradle, then its up to her. Everybody has their own priorities. She never said anyway in the Show that she cannot afford a cradle for pari. Its is what we the viewers assume. Since Onir has a steady job, I have not seen purvi having milk problems for the baby. It was only shown once that she could not afford milk and that was when Onir was out of a job and they were facing difficult times but they have both gone past that now. There situation is different. It is not fair to use the past to judge what is happening now. Life has changed for purvi and Onir and pari.