Children change your life.

When I was thirty and I was becoming a father, a friend told me: Your life is changing!
His voice’s tone and facial expression finished his phrase for him: Your life is changing. Your life will be worse!

voice's is not wrong but it's also not necessary
You could write Your life is going to change instead of Your life is changing. This gives a (slightly) stronger feeling of warning of something bad or unexpected coming.

Ten years later I admit that this true: my life changed, but now it’s better.

It’s true: without children we could traveling travel a lot; we could take every opportunity that arrives.

Michael is right that semicolons are not very common in everyday English, but in formal written English (at least in America) a semicolon is needed between 2 independent clauses without a conjunction.

However there are a lot of surprises in paternity and maternity.
I learned some things:
I learned to be more tolerant with other people.  Now I consider that people have a private life, and, probably, they are making an effort to be on their best. (To do their best is a bit more colloquial but it depends on the precise  meaning you intend.)
I learned to ask myself: why? Children ask  “why?” all the time. Their curiosity is a powerful engine of creativity.
I learned to organise: scheduling all the activities for one or two children is like a Master's in business organisation.
I learned to be present. Children are growing. They need a father, not a simple man at home. No excuses. Responsibility transformed me from a man into a father.

Practise what you teach: children do what you do. Practising what you teach is the best way to teach them. But is very hard!

So, in my opinion and experience, there is no doubt: children change your life, and they change it for the better!

Great work! And I agree, being a father has changed my life for the better. :)

PS: Your spellings of organise and practise are British; in the U.S. they are spelled organize and practice. Both are correct but one is more common on one side of the Atlantic and the other on the other side.

Children change your life.

When I was thirty and I was becoming a father, a friend told me: Your life is changing!
His voice’s tone, The tone of his voice and his facial expression finished his phrase for him: Your life is changing. Your life will be worse! (You could say - Your life is changing - for the worse!)

Ten years later I admit that this true: my life changed, but now it’s better.

It’s true: without children we could traveling a lot, we could take every possibility that arrives.

However there are a lot of surprises in paternity and maternity.
I learned something;
I learned to be more tolerant with other people: right now I consider that people have a private life, and, probably, they make an effort to be on their best;
I learned to ask myself: why? Children ask all time “why”? Their curiosity is a powerful engine of creativity;
I learned to organise: scheduling all the activity with one or two children is like gettting a Master in business organisation.
I learned to be present: children are growing, they need a father not a simple man at home. No excuses. Responsibility transformed me (from a man into a father?) to a man in a father.
Practice what you teach: children do like the same as you. Practice what you teach (preach) is the best way to learn teach them. But is very hard!

So, in my opinion and experience, no doubt: children change your life, and they change for better!

Hi Fillipo,

What a lovely piece of writing, straight from the heart and very honest. Excellent work - I hope the corrections help (not many!).

Take care,

Triss

Children change your life.

When I was thirty and I was becoming a father, a friend told me: "Your life is changing!"*
His (voice's) tone and facial expression finished his phrase for him: "Your life is changing. Your life will be worse!"  Ten years later, I admit that this much is true: my life changed, but now it’s better.  It’s true: without children we could be traveling a lot. We could take every possibility (or opportunity) that arrives. (or arises) However there are a lot of surprises in paternity and maternity.  (or parenthood)  I learned some things.  (or "something")  I learned to be more tolerant with other people. Right now I consider that people have a private life, and, probably, they make an effort to be on their best.  (I don't understand. Do you mean "at their best"? Or "on their best behaviour"?   I learned to ask myself: why? Children ask why all the time. Their curiosity is a powerful engine of creativity.   I learned to organise. Scheduling all the activity for (or maybe "activities of") one or two children is like taking a Masters in business organisation.  I learned to be present. Children are growing. They need a father, not a simple man at home (I don't understand. Do you mean: "not just a man around the house"?). No excuses. Responsibility transformed me from a man into a father.
Practise what you teach: children copy what you do. Practise what you teach is the best way to teach them. But it is very hard!  So, in my opinion and experience, there is no doubt: children change your life, and they change it for the better! 

* https://en.oxforddictionaries.com/punctuation/punctuation-in-direct-speech

https://en.oxforddictionaries.com/usage/learnt-vs-learned

This is good. If you'd like general English advice, I'd say be careful with your punctuation. Colons (and semi-colons) are perhaps quite rare in normal usage. But it's no big thing :)

https://www.grammarly.com/blog/colon-2/ 

http://www.bristol.ac.uk/arts/exercises/grammar/grammar_tutorial/page_04.htm