Need Tracfone cell phone/service alternative for young teen in England

I took an 11 yr and 13 old to europe.. when we entered crowded sights I would choose a meeting point in case we got separated.. did this with all my kids ( they all (3) got one on one trips with either their dad or me ) .. for weeks at a time in Europe.. remember.. its not that helpful to phone and say " I'm at such an such a place" if you don't know where that place is either.. lol.. so even as a back plan with a phone.. this is still a good technique to employ for all family members.
Remember people have traveled without phones for hundreds of years before about 15 years ago..

Thanks Pat. Yes, I do generally try to keep tabs on my offspring at all times and would have a meeting point plan, but I am a near professional worrier and having plan B is a comfort more than (I hope) a necessity. The thought of getting separated on the crowded Tube (not super likely I know), sends shivers down my spine. Plus having another family member with cell service is a good backup for us parents should our phones run out of battery power or the like. (Here too my worries get the best of me, I know.)

In London my recommendation is to go to a "Carphone Warehouse". They will set you up with what you need.
Very helpful people. Not knowing where you are staying and which tube station you will be using I can not give you an address. Just "Google" carphone warehouse and check their locations. We used the one located just outside Victoria Train/Tube Station.

We just had another thread about Tracfone. The poster's LG phone bought from Tracfone would not actually work in Europe even if unlocked; it didn't even have GSM frequencies, which is unusual for a modern smart phone. Tracfone has no international roaming, though, so there would be no reason for them to insist that those frequencies be included on one of their phones. Quite the contrary, actually.

Looks like the same is true for your daughter's Samsung Galaxy Grand Prime: no GSM frequencies, so won't work in Europe, even if unlocked. They use GSM over there for voice.

This is an old phone anyway. Consider simply buying her a new Android phone. Cheap Androids don't cost that much purchased outright (so could work on Tracfone or anyone - not locked). Consider the Moto phones for example. The Moto E5 play should work in Europe and costs only about $130 or so (cheaper on sale) - just be sure to buy the fully unlocked version. Then she could use it with Tracfone (or whomever) at home and buy a SIM in Europe to use it there. Take the phone to a store in Europe and have the clerk install the new SIM. It's quite easy - just keep the old one safe and secure so you don't lose it! (Bring a little plastic container of some sort - I always do.)

https://www.samsung.com/us/mobile/phones/all-other-phones/samsung-galaxy-grand-prime-tracfone-space-grey-sm-s920lzaatfn/#specs

The Moto X4 is a much nicer phone than the Moto E5 play - in fact, the X4 is my primary phone now. You can get fully unlocked Moto X4's on sale for cheap - sometimes around $150 from B&H Photo Video. They are sold out at the moment:

https://www.bhphotovideo.com/c/product/1449873-REG/moto_pa8s0024us_moto_x4_android_one.html

When landing at Heathrow (I'm making the presumption it will be your destination airport) then there are several shops alongside the baggage carousels that sell sim cards if you don't want to wait until you hit the high street.

Google "Google FI".

The last SIM I purchased in the UK was an O2 pay-as-you-go and I was very satisfied.

I like the Google Fi concept because the phone will work the minute I land so I won't have to waste the time finding a phone store. There call rates are as cheap as most of the PAYG plans and their data is competitive as well. You can get a brand new Moto G6 Google Fi right now for $99.