We are corporate expats which means that we move fairly regularly. My children are ages 8 and 10 and have lived in 4 different countries. They are incredibly flexible and know that this is what we do. Most of their friends are similar which makes life much easier. If we aren´t leaving, someone else is. I think that this nomadic lifestyle has led to them adapting quickly and integrating fairly rapidly when we do get to a new location. I must say that my daughter is a bit better at it than my son though.
They began school in Switzerland and went into a Swiss German speaking kinderkrippe (preschool). My daughter then went on to a bilingual school, German and English based on the British system. When we moved on to Slovenia, she went into an English speaking IB program and was one of the few native English speakers in her class. My son was tossed into a Slovene preschool which was difficult but doable. The problem was at week 6, when he was starting to speak the language and feel comfortable, they changed all of the teachers. This put him in a tailspin. I pulled him out and he ended up on a waiting list for the English speaking Kindergarten. Now, we are in Portugal and they are attending another English speaking school, this time in the British system. It has been an incredibly easy transition for them. Our next move will be this summer and they will re-enter the IB program (which I prefer) in English. If we were staying in countries with one language, I would have immersed them. If we were staying in one location for a long time, I would have immersed them. But with our transience, keeping the school system in one language is for the best. I don´t think it matter whether it is the mother tongue or another language, as long as it is consistent. If German would have been available in Slovenia, I would have gone for it.
So far, so good. No age has been difficult yet. I think the most important thing in our family is to keep open, positive communication. They never have heard me complain about moving or disrespecting the culture in which we are living. It is always an adventure and they approach it that way. They are actually looking forward to the next move which surprises me. Of course, they keep saying they want to live someplace where there is snow.
I also realize that all kids are different. My children are incredibly adaptable; probably due to genetics but also upbrining. Some kids aren´t. Some children can handle the big change of immersion, others can´t. You know your children, but please, just don´t superimpose your insecurities on to them. I see this all of the time. (and I am speaking generally, not to any one specific poster).