I would say it is perfectly harmless in any context but the most formal. And even in the most formal setting, I can hardly see it to be of any consequence. I would even go further and say it is systematically used by everyone who does not pay a particular attention to eloquence. In every day conversation, not using it makes you stand out.

It is, however, completely different if you intend to transcribe a conversation. Then it might come out a bit childish or disrespectful.

J’suis actually stands for je suis. This is an informal elision.

This is spoken language, so you will normally not be led to read/write this, but in certain songs or dialogues. And this is not offensive at all, this is only je suis badly/quickly pronounced. J’suis sounds a bit like “chui” and this is why you sometimes read chui on internet forums or SMS.

So can children tell their mother j’suis? As this is a matter of prononciation, actually, they do, and this is accepted or not:

— Franck, où es-tu ?
— J’suis là, Maman !
— Et bien viens vite à table, c’est prêt !

or

— Franck, où es-tu ?
— J’suis là, Maman !
— On dit Je suis mon chéri ! Faudra-t-il que je te le répète chaque jour ?