I asked the hotel concierge about going there directly from the airport, the quote I was given included the airport transfer fee of 500RMB.
Are there any public transport options between Huairou and the airport? Or is taxi the only option (and if so, does 150CNY to Mutianyu seem about right?)
Perhaps I should leave the buggy at the airport, it's just that she's going to be an ordeal to carry on the way back - she seems to gain 10kgs or so when she's asleep and my back health is not great.