its traffic at st annes free but pretty short. couple of miles Backpool is much better [its backpool to fleetwood so you can miss out Blackpool entirely but its ok on that side IME and worth it- probably would not travel from manc for either
Southport does the same and has a pier with an old penny arcade at the end of it and past it an outside exercise area for them to play on.
Give us an indication of what you want to do and how far you want to travel. There are some off road flat e tramways in Manchester or family friendly off road routes in Manc and also closer than Lytham
Top of my head
1. Preston Guild wheel – Avenham to tockholes massive play area t tockholes
2, Velodrome and Philips park
3.Route 55 sustrans – not that interesting but flat and off road and close
4. Rivvy- off road can be as flat or hilly as you want
Probably have GPS /routes of all of those