Good places to stop with kids just off M40/M25/M20?

[ThereMustAndShallBeTea]

Another thread from me, sorry, as I am on obsessive overplanner.

We are driving from Cheshire to Dover in a couple of weeks' time.

Please can anyone suggest good places to stop with kids which are close to a motorway junction and preferably either cheap or free?

Children are 6, 4 and 4 months and we're travelling midweek.

I'm looking for anywhere south of say Banbury on the M40 (DH will probably want to go further before stopping but if the traffic has been bad we may stop sooner), the bottom right hand corner of the M25 or close to Dover on the offchance we make amazing time and aren't delayed by strike action etc.

We're National Trust members so will scour the handbook also :)

Cliveden isn't far off the M40 junction 3. National Trust so would be free to get in, though food is usual NT prices.

Has a lovely play park for the children and last time I went there was an enormous metal slide whilst they fix the stairs at the main house.

Painshill in Cobham or RHS Wisley? Both easy to get to from the M25, and often have children's activities.

They aren't free though - I think Wisley is fairly expensive, but Painshill not too bad.

There is a good soft play centre in Banbury called Rugrats and Half Pints, its only 5 minutes drive from the M40 junction on an industrial estate (so you don't need to drive into the centre of the town). It's not free of course though.

The Leas Coastal Park in Folkestone, just 5 minutes from the motorway junction which I think is 1 or 2 junctions before you get to Dover Port.

These are great, thank you!

Hatfield house