I have just found out that for flights travelling after may 1st, you no longer pay tax for children aged 12 and under due to a change in government policy. If this flight was booked upto a year ago then you are entitled to that money back. We booked flights for the us and should get back over £100. Some airline automatically reimburse while others you need to contact them.
www.moneysavingexpert.com/news/travel/2014/12/how-to-reclaim-air-passenger-duty-airline-by-airline
Thought it may be useful to some as I had no idea about this.