If all of her school subjects are in Finnish her English will get weaker every year...

It is a bit different here than for your kids though. Here, every kids' English gets better every year, as it is taught in schools and of course, the main second language of the world and internet. Even now, though she speaks Finnish with them, a couple of her friends are dual mother tongue English speakers too. But, if she doesn't get enough richness now with it, she will always be a bit behind as an adult.

It will be interesting to see what happens when she finally visits Australia and talks to locals! :D

Do your kids use Russian much now?