<My wife and I both have a one year visa. We report every 90 days. We will be taking a trip in a month or so, our 18 month old came in on a 30 day visa as we heard there is no penalty for this when leaving. Is this still true? And if yes up to what age? And where can we find the law written in case we have some trouble with the immigration in bkk. Thanks for your help.

You will not have any trouble but it will likely be pointed out. AFAIK there is no law involved - it is policy. Believe the age is at least 7 and may be higher.

As you say you report every 90 days it seems you have one year extension of stay rather than one year multi entry visa so I would talk to immigration about including her. Not sure but she may need to attend kindergarten later.