What is the route for the airport express train that we should take from and back to the airport.

From the airport to Disneyland Hotel, the best way for you is to take taxi since you have 2 kids. Or you can take bus S1 from the airport to Tung Chung, then take MTR Tung Chung Line to Sunny Bay and transfer to Disneyland Resort Line to Disneyland, you will find the hotel.
From Disneyland Hotel to City Garden Hotel, you can take Disneyland Resort Line to Sunny Bay and transfer to Tung Chung Line to Hong Kong Station, then walk to Central Station to take Island Line to Fortress Hill. Get out from ecit B and walk about 500m to the hotel. Or you can take a taxi from Hong Kong Station to the hotel.
From the City Garden Hotel to the airport, you can walk to New Trend Plaza, King's Road to take bus A11 directly to the airport.