In reply to aln:
We found that the group sessions were not much use. 30 mins of next to no supervision leads to slow progress and poor technique .
Child 1 had these lessons and was disadvantaged later on.
Child 2 we gave private lessons for 45 mins a week at £14.
She has progressed to a cheaper group session that is monitored by better instructors, so it has turned out cheaper and more effective in the long run.
Don't forget to get them reconditioned to the fear of putting their head under. This slows progression in classes.
Also, take them yourselves for a splash. They may not practice technique , but water is fun.