Approx how long would you allow to spend time at the dinosaur trail and playground?
If by playground you mean Fort Knebworth, this is in a different part of the park so I think at least 2 hours to do both.
The dinosaur trail you can do in about 30mins, longer if the little ones have interest.
Top tip here is do the trail first because the kiddies wont want to leave the playground.
Take a picnic and plenty of juice and get there early, once it gets busy you get queues then the little ones get frustrated.
Cannot help with the weather so good luck.
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