Best place to meet in Paris

Les Halles is absolutely enormous and it's very easy to get lost there. Everyone finding the right exit will not be that easy.
Why not just meet at La Defense? You could meet in front of La Grande Arche on the left or right side. That's one easy place to find.

Defence is waaay out of the way. If you are going straight to Defense it makes sense but not if you are arriving earlier and have time to kill first. If you plan on checking in right away, I'd go straight to lodging and meet there.

If you don't plan on checking in until later, pick something central or base it on what you would like to do. You could meet at the Charlemagne statue, for example, in front of the Notre Dame. It's small enough to find each other but central to both Nord and Lyon stations.

Meeting indoors on a rainy day isn't a bad idea. I met someone in Paris at the Starbucks in Les Halles in September. Good thing they included the address because I passed at least two other Starbucks before I found the one we were supposed to meet at. If you pick some place big, be really specific on where exactly you will meet. Hopefully, you will have communications (that's why Starbucks was a decent idea, they have WiFi so I could use my phone before I got my local SIM).

To make things easy for your daughter, why don't the three of you meet her at Gare du Nord and then all take a taxi to La Defense? Taxi fare divided by 4.

Chatelet is the worst possible meeting place. It is enormous and there are over 20 exits. It will be a confusing place to connect. Note that you need to take the metro line 1 if you will be going by train unless you buy the more expensive RER ticket to la Defense. La Defense is not in Paris and thus costs more on the RER; the metro is one price even where it goes outside Paris so an ordinary ticket works on the metro.

I would meet at Gare du Nord and take a taxi if you want to go together. Otherwise, it would make more sense to meet at the hotel. Better to be relaxing at the destination than standing around in a train station waiting for other people. If someone is going to be waiting around at a train station be sure they have valuables well stowed, preferably in a money belt under their clothes and are keeping a good hand on their luggage especially smaller bags or purses.

If you want to meet her, take the RER D from Gare de Lyon to Gare du Nord. It's one stop and takes 7 minutes total.

If you don't want to do that, You can meet on the RER A platforn at Les Halles, Direction La Defense. But the platform is long, so do tell her either head of the platform or tail of the platform. This is one way Parisians meet up, on the platform in the direction they are going. There will be signs saying tête du train (head), train court (end of a short train) and train long (end of a long train). Chose tête or court and meet there.

To get to the RER, just follow the overhead signs that say RER.

How far ahead of time are you arriving? If you are just going to La Defense to drop off your stuff and head to the concert, why don't all of you just meet in the lobby of your hotel? If you have a lot of time before you head to the concert and want to hang around in Paris proper first, I would not meet at Chatelet-Les Halles but if you are taking Line 1 to La Defense, meeting at Chatelet station would not be difficult. You could just agree to meet at the middle of that platform. Your daughter will need to change from the RER B to Line 1 at Chatelet-Les Halles and make her way to the Chatelet stop on Line 1 to get to La Defense via Metro anyway and meeting in the middle or head or back of that platform is easy. You would get off at Chatelet and wait on the platform in the agreed upon area for her. I usually choose the middle of the platform so I do not have to figure out which direction the train is traveling. I wouldn't do this meeting at Chatelet if you are traveling during rush hour. But anyway, unless your scheduled arrivals are within a couple of minutes of each other, instead of all that planning why don't you have whoever is scheduled to arrive first meet the others at the train station.
You may know this already but if you are using a regular t+ transit ticket for this trip, you would have to take Metro Line 1 to La Defense so your meeting point would be the Chatelet part of the complex.

Wow thank you folks. Lots of information. Having read them all I will take most people's advice and meet her in gare de nord. Easier all around especially with the train leaving directly to gare de nord in one stop. We can have lunch together & move on to our flat to dump bags & off to the concert.

I have another question does anyone know how much a taxi costs from gare de nord to La Défense? Approximately of course.

The trip by taxi is around 30 minutes and will depend on the time you are leaving the station. Estimates are 35 EU to 75 EU, if in heavy traffic.

Important - In Gare du Nord, go directly to the line where the agents "shoo you in" to the dedicated taxi service. Do not go with anyone who walks up to you.

If you are going to a hotel, you can plug in the address of the hotel and go from there:
www.taxifarefinder.com/

The exit for the legitimate taxi line at Gare du Nord is on the west side of the building. Every story that I have ever read about a taxi rip off at Gare du Nord either begins with following some who approached the traveler or getting into a taxi that was found in front (south side) of the station.

Now the agents direct you to the dedicated taxi stands. You do not have a choice.

Look up a McDonalds or Starbucks near Chatelet Metro. Sad I know, but easy and convenient if your wait becomes exacerbated for whatever reason.