Here in Belgium we have to give vitamin D up until they are 5 years old!
I have a little dropper which I give every morning after she wakes up, lying next to the nappies so I don't forget.
I don't give them to my eldest anymore since she takes multivitamins now instead.
But just like you, we have more rain than sunshine so it's important for bone growth and strength. A certain disease is coming back from the dark ages since children spend more time indoors on their computers, consoles, tablets than outside. I can't remember the name but it started with an R. Rachitis? Not sure.
But I know vitamin D helps prevent it!x