T-Mobile offers unlimited 2G (slow) data when traveling internationally. The data allocation of your US plan will be irrelevant when traveling. If you want data faster than 2G, you'll need to buy "data passes" from T-Mobile while you travel. The data speed may be fast enough or it may be frustratingly slow. (While on WiFI, this is not a factor - your phones will be as fast on WiFi as they are at home, assuming the WiFi you use is fast.)
Make sure you don't have an old T-Mobile plan that doesn't allow international roaming. If it's "Simple Choice" or newer, should be fine; call T-Mobile to confirm.
Calls are 25 cents/minute while in Europe with T-Mobile. If your phones support the "WiFi Calling" feature, you can make free calls home to the US while your phone is connected to WiFi. If not, you can install Google Hangouts on your phones. This will let you make free calls home to the US, even to landlines (say to call your bank or airline or whatever - handy when you might be on hold for a while and not paying 25 cents/minute for that).
They give you free unlimited texting as well.
You won't need to buy a new SIM if you stick with T-Mobile. You CAN buy a SIM in Europe for the faster data, if your phone is unlocked. A European SIM bought in one country may work in the others (depends on the mobile company and country you buy the SIM in - many allow free EU roaming, some don't allow it at all). If you have a European SIM in your phone, you won't have access to your US phone number until you put your T-Mobile SIM back in.