only 9 minutes transfer time in Brussels MIDI

Tight, but doable. 15 minutes would be comfortable
Must train websites allow you to specify a minimum connection time, for example if you are slower than the average person, or want time for a coffee/sandwich.
Two different rail companies does not make a difference, it is the same station.

If it is one ticket, you should be covered if the first train is late, and allowed onto the next train to Amsterdam.

Thank you very much for your advise

Check out this page on Bruxelles Midi station:
https://www.seat61.com/stations/brussels-midi.htm

Helpful advice on how to make tight connections using the mid-platform connection tunnel ("couloir sud").

Since both trains require a reservation, you'll know exactly which train you need to be on. Is this booked as one ticket, or did you combine 2 tickets together?

if you have combined the tickets yourself, I would say it is a bit too close. If it is booked as one continuous journey you should be OK. There is a Thalys train every hour, and customer service would be able to get you onto the next if through no fault of your own your first train was late.

Thank you very much all!

If you can get a later train, I would. I haven't been to Brussels train station yet, but unless you know exactly where to go you could easily miss the second train. It is a major station after all.

Brussels South is a modern well signed station. With as many platforms as it has it is fairly compact. All trains arrive and depart from the floor above the main concourse so it is off the train, down an escalator, over to the escalator you need and back up and on the train.

Thalys is usually well labelled and signed - look for their reddish purple logo.

First of all, thanks to all those who give help and advice. We consulted our questions on the Facebook of thaylis a few days ago. The answer was as follow
The platforms where the TGV trains arrive and those of Thalys departures are not far from each other, usually.
We can unfortunately not know in advance which platform will your arrival/departure be, as it changes all the time, but you can ask the Train Manager during your journey to Brussels to check which platform is your Thalys connection at.
We usually advise our customers to book a later Thalys, since 9 minutes can quite a short time or even late, especially if your TGV connection arrives with a few minutes late in Brussels.
However, if you decide to go by your plan and in case you miss your Thalys due to a TGV delay, do not forget to get a stamp by the TGV Train Manager as a proof of delay or enquire art the international desk in Brussels-Midi station.

So we gave up the train ticket from Lille to Amsterdam witch only took 9 minutes to transfer, bought two separate tickets, and tickets for Thalys an hour later.

That will relieve the stress... Perfect reason to get some good chocolate while you wait. There are several stores in the station.

Chocolate relieves the stress too, you know, and helps with any dementors you come across on the trip...

Great！ It’s a good idea to buy chocolates while waiting for the next train， Thank you very much!