Stop offs between Heathrow and St Austell

[Mumofcats5]

Wise MNers
Later this month, I'm travelling with DH and DS to St Austell from Heathrow area. DS will have just turned 2 - I suspect that the 4 hour or so journey won't be so good for him so I'm seeking advice on good places to stop en route - we have National Trust and English Heritage memberships.
Thanks in advance!!

Stourhead (National Trust) is probably about half way and very close to the A303.
Hunter’s Lodge at Wincanton is only one minute’s drive off the A303 and is very popular with families as it’s ideal to let the kids let off steam in the excellent, huge playground. It has a verandah and terrace ideal for outdoor dining.
If the weather’s bad, try Mini Town at Cale Park Kitchen

Thank you @Ricekrispie22 excellent suggestions!!