Ethiophil
The Sandstone Trail is 34 miles in length. Only you will know the ability of your children but Frodsham to Manley Common is 5.5 miles of moderate walking. It does have some steepish ascents and decents, there are some narrow paths and caution with children in some parts may be required I would suggest. I would say stretch from Frodham is probably the most interesting with some stunning views across to Liverpool and the Mersey, plus North Wales, and there are numerous points of interest on this stretch. It is well waymarked so shouldn't be too many issues there. OS map Northwich and Delamere Forest would be useful to have - a guidebook wouldn't go amiss, we have Walking Cheshire's Sandstone Trail (Tony Bowerman). As for cattle, it is Cheshire, there is farmland and so you never know - depends where the farmer puts them at any particular time. (not had any issues on this particular stretch personally).
For me I prefer this walk in the Autumn with the changing colours, each Season is different and really it is a matter for you.........
FL