How to say "Paris is a good city in which to raise children" in conversation?

Paris is a great city to bring children up in.Paris is a great city for bringing up your children in.

Note: in the UK we more usually talk about 'bringing up' children. 'Raising' them is more American. Both are of course perfectly OK and interchangeable.

If raising children is improved by a good environment then I would say that Paris is a good place to raise them.

Paris is a good place to raise children.

You are mistaken. Many people would say that. It’s only “formal” if one’s speech is otherwise sloppy.

I told you before- "Paris is a good city to raise children in." That's what most native speakers would say. 

Since you are studying English and actively seeking advice on the language, please allow me to point out a subtle difference between "learnt" and "learned" (not on point to your question, but contained within).

IF you are speaking English in the US or Canada, then English speakers there are more likely to use "learned."   They would tend to try to correct your usage of "learnt" although you are not wrong.  US/Canada English speakers more naturally use "learned," where as most of the rest of the English speaking world tends to favor "learnt."  So - remember your audience.   =_)

On point to your question:  "Paris is a good place to raise kids."   

Most people understand Paris is a city and kids is very natural and informal.  

Hope this helps you.   

Paris is a good city to raise children.

You can leave out the "in which".

Paris is a good city for raising children