Good news is that children age under 11 go free on the Underground, no proof of age required. If you have contactless credit/debit cards (one per adult) use those for your travel; otherwise buy one-day travelcards at Croxley Station. When entering or exiting tube stations, one of you should use the wide gates and then the children can walk through with you. You will get the hang of it!
Thank you. I wasn't sure whether heading to Croxley Green (which is closest ) to the hotel would work best or if we'd be better heading into Watford centre and go from Watford Junction?
The Premier Inn is probably equidistant from Watford and Croxley stations. Either involves a walk along the busy A412 and if you want a scenic option, walk down Blackmoor Lane and cut through Byewaters (it's a newish residential estate) and walk along the paved path next to the canal. Cross the bridge next to the lock, take the track up the hill and you come out on the A412 opposite Croxley station. The only traffic will be narrowboats.
There's also a beautiful SSSI beyond the lock where the Gade and Grand Union Canal run side by side, if you want some green space and wildlife. Swans, water voles, kingfishers and damselflies ... it's a great place for a picnic or a walk. It's a world away from the busy road junction the Premier Inn is on.