I think so much depends on the personality of the child. Your younger one would be fine starting off in a Spanish school, but we have no regrets (apart from the cost!) about opting for an international school for our eldest who was just 9 at the time (last Dec). She found the move itself quite traumatic and upheaving enough without coping with a new language, and also we were able to liaise very closely with the school through this time till she settled. I couldnt imagine how we’d have coped at a Spanish school if only because OUR Spanish wouldnt have been up to it.
I do know a lot of kids our eldest’s age though do go straight into Spanish school and are absolutely fine. Kids are generally extremely adaptable and flexible (ours too just took a little time!) Our 4 year old took everything completely in her stride. And we are very happy with the outdoor and family friendly lifestyle we have here, which makes all the adjusting so worthwhile.
Definitely do your homework, you need to find work you can rely on and decide exactly where you want to live! Whatever school you choose it will help your 10 year old immensely if they can visit first on a holiday and meet some potential friends and teachers, also encourage email exchanging with friends met on school visits so they can ask their own questions about what is of biggest concern to them,
good luck!
Maya