There are no trains between Gare de Lyon and Brussels. All trains from Gare de Lyon head south towards Lyon, hence the name. On the other hand all trains heading north leave from Paris-Nord.

Simply take RER D between the two stations (not included).

Thank you for your help. Do you know if its possible to change your departure time?

Should be possible with Thalys seat reservations. On your seat reservation, there should by a booking (PNR) code and your name/e-mail adress.

https://www.thalys.com/be/en/deals-fares/eurail-and-interrail-passes-with-thalys

On the Thalys website, go to the right corner, click tickets and fill out the details. You should be able to rebook there. If not, best contact Thalys or the place where you bought seat reservations.

Pass holder seats are limited on Thalys, if a train has already run out of passholder seats, you won’t be able to board.

Why do you want to rebook? You calculated not enough time for the transfer between Paris Gare de Lyon and Paris-Nord? Best provide full details. What’s your full route that day you take the Thalys from Paris to Brussels? When are you travelling, how late, which date?

Yes I think it’s going to be a rush given we are travelling with three children. 
We are staying in Le Lilas so need to get back to Paris Gare Lyon then to Paris-Nord!

is this a reasonable time frame?

You didn't mention any times, so impossible to answer.

You didn’t give a time frame

You mean the Les Lilas, a suburb of Paris?

That’s actually closer to Gare Du Nord than Gare de Lyon...

Just provide a bit of extra time to take the metro in Paris to get there (M5 or M11/M5). Use google maps to guide you and try to arrive 30-40 min in advance of your train leaving, if you’re stressed about not getting there in time.

Apologies. Yes Le Lilas a suburb in Paris. 
Our train to Brussels leaves at 1025 am from Paris-Nord. 

Than I don’t see a problem. No need to go to Gare de Lyon.

Platforms show only 20 min prior departure in France. Boarding only starts then too normally. If you try to arrive at 10:00 Gare du Nord you’re good.

Use https://citymapper.com/paris to navigate around Paris (and any other big city in Europe).

Metro, RER are not included in your pass, but it’s quite cheap in Paris anyway.