A train adventure?
Oh! You mean the journey there?
Manchester has some excellent entertainment for children.
In the city centre:
Museum of Science and Industry ***
Manchester Art Gallery (includes a dedicated children’s floor with interactive stuff).
The Whitworth Gallery
The National Football Museum
Manchester Museum - full of wonders.
Take the tram (great fun for littles) to Salford Quays. Walk along the canal side (where there will probably be water sports to observe) to:
Imperial War Museum North - so much to engage with.
See if there’s a show on at:
The Lowry
Or something child friendly going on at:
Media City
In addition there's a proper steam train adventure available from Bury Train Station. Google and book in advance.
(In truth I’d say Liverpool has more - but I guess eventually you’ll do both.)