The EU law is quite clear. USA GMRS and FRS is not applicable.
You can have a PMR446 radio, with eight channels and subcodes beneath, 500 mW and fixed antenna to limit erp.
It must be type approved and marked PMR446 to be legal.
That said, plenty of us know how to program e.g. Baofengs etc, on the proper frequencies, at illegal powers, with good antennae, and get good use from them.
There are also a further 8 PMR446 channels assigned to digital use, which never caught on. Some use those too, analogue, to avoid congestion.
Then, in the UK only, there are three frequencies assigned to “UK business light” which are uncongested, allow higher powers and within range of typical re-programmed things. More frequencies also for e.g. re-programmed marine VHF or 2m/70cm band amateur radios. To be legal you have to buy the license (no test), for unlimited handsets throughout the UK, costs about £75 for 5 years.
If you move beyond the assigned 16 PMR bands (8 analogue, 8 digital), or the UK only “business light” ones, expect to be monitored and maybe even investigated.
Stick with legit. PMR446 radios, the non-toy ones are good, and have enough range for many purposes, especially children. They can be a lot of fun.
Otherwise “citizens band” 27 MHz transceivers are legal and license free across Europe, though frequency bands and modulation (AM, FM, SSB) may depend on country. More expensive, but in the right conditions can propagate across continents.