I'm a teacher and if I was relieving I would get around $200 in the hand per day but I would expect more if I was teaching a specialist subject like yours. I was looking at providing Professional Development for teachers and would be charging about $250 for half a day, and thats with no homework or marking to do.
If we go on a trip then it usually costs anywhere beteen $4 and $15 per child depending on what they are learning about (not inluding transport). Music lessons are usually betweet $20-$40 if they are in school per child, depending on the subsidy/instrument.
Next term our kids are going to swimming which will cost about $20 per child for 5 sessions including transport.
I would be looking at charging $20-30 per child per session, as you will be doing a lot of prep by the sounds and will need to factor in that time which will probably be about an hour per session if you are giving homework etc. The school will soon say if it is too much for them to pay.
Hope this gives you something to compare to