The reason the choice is limited to these two is that in general, children of this age are physically too small to play many other instruments, and their lungs are not large enough to cope with most wind instruments. But, as with any musical instrument, skills learnt on one are quickly transferable to another.
piano; violin; 'cello; flute; saxophone; clarinet; drum-kit; guitar (classical); trumpet; trombone (new this year)
There is a different rate for some instruments, depending on the size of the teaching group. Prices per lesson are as follows, and you would expect to receive 30 lessons over the school year:
Violin (groups of 2) - £7
Piano (one-to-one) - £14
Piano (one-to-one) - £15
Drum-kit (groups of 2) - £12
Everything else (groups of up to 4) - £7.50