Where is the best place in Europe to spend winter training?

I'd head out to the Spanish east coast (Catalunya, Valencia etc.). Lots of tracks, lots of riders from all over Europe (slow riders and all the way to top MXGP riders).

The minus is that there aren't many sand tracks. Some of the Red Sand tracks can be a bit softer and deeper on some days but definitely not proper sand.

Slava Ukraini!

Northern Italy. In the Verona and Milan area there are literally dozens of tracks.
Only thing to note is that some tracks are very easy (Crotta d'Adda, Dorno for example). Great for beginners or just-for-fun riders but not so great if you plan to train for EMX. There are lots of others though where you'll find the better riders (Cremona, Mantova for example). Lots of sand, lots of hardpack and everything in between

-If you are going for a MXGP career, put them in the northern sand. Lommel in Belgium. Grevenbroich in Germany. Arnehm, Oss, Valkenswaard, Berghem in The Netherlands.

-If you want to make them complete riders, then this and the Murcia area in Spain, as mentioned already. RedSand is the most famous, 3 tracks, one is deep red sand. Only open in the winter though. Around you have solmoto, mxgolfpark, vallgorguina, verdegas, Totana, Rocco's ranch.... and a supercross right next to Murcia.
There is a British company that specializes in MX holidays and they offer training with Tommy Searle. Check them out. Spanishmxholidays.
There is also a spanish app MXCIRCUIT that lists every spanish MX track.

In both areas you will see a bunch of pros and EMX and MXGP teams in the winter, testing.

South of France could be good too. CDGmxretrat, Royal Hills, MC des Costieres (prepped by Greg Aranda), La Fare (SX track), Draguignan, Saint-Thibery, Chateauneuf, Valence,...

Personally, I think Italy is more for fun or beginner riders, as mentioned. Tracks are prepped everyday and they are easy. Nothing good for a serious training. Plus the northern area is quite dirty, a lot of abandoned buildings and farms, not very pleasant to see, but that's just my 2 cents.

Most of the UK guys hit Spain now, loads of MX and supermoto guys down therefor winter training, loads of tracks and its really cheap. 

We did Northern Italy , with my German based team, Went to Dorno and Ottobiano which are about 60KM apart i think,  Ottobiano is decent sand, Dorno is a little less, and its a lot nearer to you as well.

We have guys who will transport bikes, and riders fly in and out, makes it easier. 

Spain it great but its a long way for you. 

Lommel is great but this time of year could be a bit too much for the youngsters with confidence. 

The problem with Belgium/Holland is that it’s so close to sea level so when it rains the ground water level rises and the tracks get flooded and you have to go to germany to ride instead.

If we are talking about 65/85 riders, being able to ride and get hours on the bike is the most important thing.

i would look for a place that is weather neutral, have good facilities etc and when the kids start riding 125 focus on the sand tracks in Belgium.

Is there an MX track on Malta? 

GOZO MOTOCROSS ASSOCIATION    https://www.gma.org.mt/

I have nothing to add but just fascinated at the talks of where to ride in Europe during the winter. Sounds amazing. I should leave Minnesota some winter and go across the pond to ride 

If you can make the trip Sardinia could be an option. You have both sandy and hard pack tracks, they're often left unprepped. Many pro and fast riders, kids too, go there for training over the colder months. The weather usually is good and it doesn't get as cold as it does in Northern Italy.

Central Italy is very good too: not many sand tracks, if at all, but many technical and hilly ones. Plus, the food is amazing.

I would also recommed Spain as other users already mentioned.