I am just starting my own business holding Lego Parties in peoples own homes. I am totally unsure of what to charge and how. I.e charge per child an hour or just by hour. The cost of the Lego is high, for instance a master build session for the Lego alone is already £5 a child, with a party bag add on at least £1.80 and if you want a minifigure in the bag another £2.49 on top. That is without my time added on. How much would be reasonable to ask for an hour. I would take charge of the children for the hour so parents can relax and chat with friends if they don't wish to join in. I keep fill flopping on my pricing and am driving myself mental.
Lego engineer sessions where we live are £10 for the morning per child that is for three hours. That is not a party though. Problem with a one hour activity is parents still need to entertain the kids for another hour of the party. Can you make it two hours and do the whole party? Would be better financially for you as well, one hour doesn't seem worth your while, unless you are able to do lots of parties in one day. When I have booked children's entertainment of any kin I have usually paid a set fee e.g local entertainer is £120 for two hours.