currency exchange, cell phone usage..

Currency exchange will cost you a lot of extra in their exchange rates. The best bet is to use ATMs, just like you do in the US. Be sure to decline them billing you in US dollars as again they do not give you a favorable rate. Have it billed in the native currency, euros in Paris and Belgium and pounds in London.

Others can answer you better on the cell phone. I have an unlocked Samsung so I just get a local SIM card when I travel.

ATMS are the way to go, but be sure that you have notified your bank before you leave of your intention to travel and where you are going. I would make sure that I have a back up for every card.

You can facetime with each other and with others as long as you have a wifi connection. Wifi is available in hotels and many other locations. You should be able to manage your email through wifi.

Whatsapp will be really useful for texting/calling your family/friends during your trip but make sure everyone with whom you want to communicate has the app, also! Probably your cell phones are "unlocked"- if you are not sure you should check with your provider prior to leaving. Upon arrival, before leaving the first airport you can buy a basic SIM card that will work within the EU. If it's a staffed kiosk/ store ( probably, with your major airports) the agent will likely help you install it and set it up. If you are not sure how, before your trip, it's very easy- look on YouTube or go to a phone store. If this is all new, make sure someone also shows you how to use these wifi/data settings so you are not having to figure it out on vacation!
I only used mine for occasional navigation or quickly looking up opening hours and have never had to add data in trips over 3 weeks long. (Just be sure to put your usual SIM card somewhere safe!) Use wifi whenever possible, in your hotels, museums, etc but make sure it's a secure (password protected) network for checking email, etc.
Currency: most responses will advise using ATMs upon arrival for the best rate. Make sure you notify your bank and/or card company but some have fees. I am the outlier, and purchase several hundred dollars of the relevant currency prior to departure. Why: I don't want to worry about it upon arrival and in the... event of some emergency...My last two trips I only ever needed it for small purchases like ice cream- then my final day I put it all towards the hotel balance so as to not have any left.

Start with Rick's excellent pages on these topics.

Here's his page of money tips. Read all the links, and you'll be an expert: https://www.ricksteves.com/travel-tips/money

Here's his page of cell phone tips. This topic changes quickly, and there are lots of variations for different individual situations, so you may need to come back to ask specific questions (post a new question if that's the case): https://www.ricksteves.com/travel-tips/phones-tech

One caveat. Make sure you use bank ATM’s. Privately owned ones will give you a very unfavorable exchange rate.

Do the research that Harold suggests. In 15 years of going to Europe yearly, we get European currency at our arrival airport there via an ATM that is connected to one of the networks our Debit cards belong to. Be sure not to use an ATM machine connected to something like Currency Exchange as they have the worst exchange rates we have ever seen. We get enough local money there and then later after getting settled into the place we are staying, we go out and find an ATM machine connected to a bank that is open just incase the ATM machine eats our card which has never happened. We always take two debit cards from different financial institutions as well as two credit cards also from different financial institutions thus have a back up just in case. Only had to use the second debit card once.

With iPhones, you can use iMessage and FaceTime instead of What's App (as long as you are contacting another iPhone user).

Install Google Hangouts on both phones so you can make free calls home to the US, even to landlines (to call your bank or airline in a pinch, without having to pay per minute for calls on a SIM card). Add a +1 to the front of US phone numbers to call them from Europe. I'd install Hangouts before you leave the US and test it out - should work the same in Europe other than adding the +1.

But you probably want to buy a SIM card in Paris for each phone if you want to use it while not on WiFi. If you don't mind using the phones only when you have WiFi at the hotel or a restaurant or something, no need to buy SIM cards at all - your phones will work just fine. You can make calls with Hangouts on WiFI if need be. Calls to non-US numbers cost a few cents per minute. WhatsApp is very popular in Europe so you might install that too if you need to contact people in Europe - many of them will already have WhatsApp. Again, you just need WiFi for that.

With iPhones, you can use iMessage and FaceTime instead of What's App
(as long as you are contacting another iPhone user).

Only if you have wifi or data coverage