how to communicate in Brussels and London

Check out Cellular Abroad. Not cheap but easy and you'll gave the phones b4 you leave home.

none of our phones will work with a sim card bought when we get there...

Are you sure about that? What kind of phones do you have?

Certainly the simplest solution would be to buy a local SIM for each phone, so before you rule that out, you will want to be sure that's really not an option.

Cheap phones (not smart phones) start at about €20/£20

See these two pages from Tesco - one of the UK's big supermarket chains.
Phones: https://www.tesco.com/phones/shop/pay-as-you-go-phones
Pay-as-you-go SIM cards: https://www.tesco.com/phones/shop/pay-as-you-go-sims

Any phone bought in one EU country will work in all EU countries. It will have a phone number from whichever country you bought it in.
Be sure you know how to add credit to the SIM card over the internet before you leave the shop.

I'd check with your carrier. We went with AT&T limited texting and calling while in Europe and served our purpose-since there is WiFi in many locations nowadays.

Some phones (usually those being bought on a contract) are locked to the cell provider that the purchase contract is from. And many providers won’t unlock them until they are paid off.

My advice is to buy a cheap smartphone in your home country – possibly cheaper and you will have time to become familiar with it.

What do you need mobile data for? You can survive on Wi-Fi if push comes to shove.

There used to be (as recently as 2015, at least) technical differences between the technology used by European cellular providers and some US companies. A lot of Verizon phones wouldn't work in Europe. I do not know whether that is still the case, but it's something you'd need to check out before just snapping up telephone hardware in the US before your trip. The issue of phones being locked if they are obtained from a US cellular provider is also real.

A smartphone app like WhatsApp will allow you to text back and forth in Europe without any sort of cellular service, as long as you have access to Wi-Fi. These days, many museums have Wi-Fi, and it's common in restaurants as well. I have run into issues on occasion in Europe, when a restaurant or train/bus company required me to register for Wi-Fi access and then texted an authorization code. In earlier years, when I didn't have a SIM usable in Europe in my phone, I couldn't receive those texts, so I sometimes had no way to use the Wi-Fi that was supposedly available.

I really enjoyed having data access in Europe this year. My trips are long and I spend a lot of time on trains and buses. I was able to keep up with this forum on my phone. However, when traveling solo in previous years, I got around fine 99% of the time without a SIM usable in Europe inside my smartphone. I just used Wi-Fi for Internet activity, mostly after returning to my hotel in the evening. The only problems I had involved some small lodgings that didn't staff their reception desks all day long. I needed a way to contact them when I arrived and found the door locked. That was awkward without any sort of phone service.

Non-solo travelers who are not going to spend 100% of their time together may well need a way to get in touch with each other. WhatsApp is what my travel companion and I used this year, because she didn't want to get a European SIM for her phone, and her cellular provider charges way too much money for roaming. WhatsApp is very easy to use.

The subjects of phone and data service come up often on this forum. It would be worth your while to read some of the earlier threads and then come back with questions. Just scroll back through this Technology Tips forum.

Well, I admit I'm 68, but people did make successful group trips to London and Brussels before cell phones existed! If the trip isn't too long, I wouldn't spend $50 a person to be able to text "B thr sn" for 8 days. It is true that my wife and I once got separated on a Hungarian tram line by closing doors, but neither of us ended up in jail or abducted.

I am the same age as Tim, and I agree, but some folks are more nervous about being out of contact than others. And this may not (one hopes) be the last trip to Europe, so the phones may end up being used for more than 8 days.

dg,

It would help to have more information. For example, which cell network are the three of you with, and do they offer international roaming? What kind of phones are you using?

There used to be (as recently as 2015, at least) technical differences between the technology used by European cellular providers and some US companies.

Google GSM versus CDMA cell phone technology, and there are a lot of articles on Wirecutter or at pcmag.com about the the different standards. European carriers use GSM as do T-mobile and ATT in the US. Sprint, Verizon and US Cellular use CDMA which is not really compatible. When I lived in Europe a few years ago, I had a Sprint iPhone that with global roaming (free) handled things ok but not very quickly.

Question 1 - Who is your current carrier?
Question 2 - Are any of the phones GSM enabled and unlocked so you could install a new SIM card bought in Europe?
Question 3 - Your phones should work with WiFi, so how much will you need to use them otherwise?

For only 8 days - buy three cheap phones (10-20 Euros each) for local calling and texting (they will be dumb ones - but will handle text fine - incoming texts are free even if you are out of minutes). Use your smart phones with wifi.

"European carriers use GSM as do T-mobile and ATT in the US. Sprint, Verizon and US Cellular use CDMA which is not really compatible."

That's not as much of a problem these days as not only are the larger networks using LTE or VoLTE, but many of the newer phones are capable of using different technologies. However if the OP is using one of the smaller regional carriers, they may indeed be still using the somewhat outdated CDMA which won't work in Europe (or most other places in the world). The world is moving to 5G and those that continue using older technology will be left behind.

Hopefully the OP provides some further information.

So much clarity in the Original Post "none of our phones will work" and so many who don't believe him.

The answer is probably around 150€, maybe as low as 99€ per person tto buy cheap smartphones in Brussels and use there and in London.