I wouldn't stay in the town in peak season, it's full of very noisy drunks until the small hours, but that is only the centre. Never stayed in Hendra so can't comment on that. Lots of nice rental properties around outside Newquay though and lots of lovely beaches all along that coast.
For kids try The Eden Project, the aquarium in Newquay and possibly the shipwreck museum in Charlestown if they're into pirates. Ours would go on the beach in all weathers given half a chance though!
Agree with bluey. Newquay is not really geared to kids bucket and spade holidays any more. soon as gcse finishes at start of July through to uni starting end of sept it is full of teenagers staying in surf shacks. there are also quite a few noisy music festivals so suggest you google this and check when they are. Headland hotel is nice. friends went this year in school summer hols with 2 dc's and didnt stay in Newquay once. Polzeath nearby has a fab beach or better still try the south coast if you havent booked accom etc yet. easier to get to (avoids A30 car park sorry road) waves not as rough ( less surfing =less teenagers). we go to SE cornwall a lot so pm if you want info.
What's your budget? Watergate Bay is just outside Newquay. Gorgeous family hotel on its own beach, Jamie O restaurant on site, tons of activities, children's meal times. Everything laid on. It is pricey but out of season they do good deals.