Kendal, Cumbria
A working farm on the Lakes' southern edge that cuts a giant maize maze fresh each summer - open once the corn stands taller than the children, roughly mid-July to September - alongside karts, trampolines and animal handling.
The maze is the event; the farm park makes it a day. If you are heading up the M6 to the Lakes, it is barely a detour.
Looking for more farms & animals near Kendal?
A medieval tower house with proper gardens and a wild play trail in the woods just off the car park - climbing wall, rope swing, stepping stones. For National Trust members it is the easiest Lakes stop off the M6; for everyone else the garden ticket is the sensible buy.
A small zoo five minutes off M6 junction 35 with over 100 species and, crucially, a lot of it under glass: tropical hall, butterfly house, aquarium and small mammals inside, dik-diks and hornbills out. Compact enough that little legs see everything, open every day from 10am.
A free 12th-century ruin on a grassy hill above Kendal, open all the time, with walls to circle, arches to charge through and the whole town and fells spread out below. Ten minutes of steep pull to get up, then let them loose.