1. Number 2 is the best wording.
2. "Breeding" is about animals, not people.
3. "Cuz" is not a word.

Student or Learner
Source : My own writing practice
1. My city is baby & kid-friendly designed cuz there are many young couples in the city who breed babies.
2. My city is well-designed for babies & kids cuz there are many young couples in the city who breed babies.
3. My city is kindly designed for babies & kids cuz there are many young couples in the city who breed babies.
Which would be the best option among these?
1. Number 2 is the best wording.
2. "Breeding" is about animals, not people.
3. "Cuz" is not a word.
In writing, unless you are reporting speech, use "because". We use neither "cuz" nor "cus" in BrE. In brief messages (texts, WhatsApp etc), you'll see "cos".
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.
I meant it in colloquial English, so the only choice seems to be 'cause.
You can simply say there are a lot of young people with children.
Last edited by GoesStation; 12-Nov-2019 at 15:38. Reason: Fix typo.