8 hour layover in Heathrow w Kids

Stay put. Not worth taking the chance of missing your connection.

• How long does your airline (Aer Lingus) say you need to be at the airport check-in (or security if already checked-in) before your second flight? This depends on the destination and airline.
• Do you need to retrieve your bags and check-in again, or can your bags be labelled through and get your second boarding card before arriving at Heathrow?
• If not, can you do check-in and deposit your bags before leaving the airport, or will you have to carry them around? Some airlines don't let you check in more then x hours in advance (value of x varies with airline).
• There is no customs on exit (and minimal customs on entry). What takes time is (on landing) taxiing, de-planing, walking through corridors, immigration (10 minutes to 1 hour +), getting to transport.

All the above eat into your time window (08:00 to 15:30 = 7½ hours).
All said, I think the best option is to go to Windsor. A castle is always kid-friendly, the town is nice and it is close enough a taxi both ways won't break the bank.
Central London would involve over an hour each way, just to see the outside of one of the sights.

Based on last month's trip, Security in the UK is taking longer than ever . . . but your experience may (just maybe) differ, and the time of day, day of the week, and exact day of the month can have an affect. For what it's worth, another poster earlier this week had a shorter layover than you, and was heading home to the USA (necessitating a 3 hour check in allowance), and one suggestion was if they truly wanted to get out of the airport and see a bit of Britain rather than the inside of the terminal, then heading to Brighton was worth a quick visit. If your kids saw Windsor Castle, while it isn't be Buckingham Palace, it would a royal residence. And, there's a very slight possibility (our flight once flew over downtown London prior to landing, but it hasn't happened just after taking off from Heathrow), you might spot the London Eye and/or Buckigham Palace from your plane!

Brighton is the obvious nearby answer for Gatwick airport.
Windsor is the obvious and even nearer answer for Heathrow airport. Brighton from Heathrow does not make sense.

Windsor is a 15 minute cab ride from Heathrow, you could go there. You should be back to Heathrow by 1:00pm.

Melissa,

I've never attempted to do what you are considering but here's my input.

When arriving at a London international airport (at least Heathrow), arrivals with a destination of London/surrounding area go through one set of passport control lines while those arrivals merely transiting the airport and connecting with another outbound flight go through a different set of queues. I've always been asked the "what, where, when" questions by passport control before being allowed to pass to the "land side" of airport. I'm not 100% sure they will allow you to process into the country if they know you have a connecting flight in less than 12 hours.

At any rate, I also agree that an 8 hour layover isn't enough time to leave the airport for any worthwhile sightseeing. Plus, the Heathrow Express isn't exactly cheap and buying multiple tickets for just a couple hours of sightseeing might be pretty extravagant...

Melissa,

I really don't think you have the time to go visiting while waiting for your flight at Heathrow. I have been through Heathrow three times this year and it seems to take longer than ever to get through passport and security checks these days. As someone else has mentioned, if you are in transit for a connecting flight, then you will be asked to following the purple line and you will go through a different passport control area. However, It is possible to go through the regular passport control to go Land side and then enter back into your terminal at a latter time, but be ready for a lot of questions at passport control if you decide to do this. (They really frown upon people doing this). With 8 hours it would be possible to visit Windsor for an hour or two if everything goes quick and smoothy at Heathrow. I am not sure I would be willing to add more stress and anxiety into my traveling day and I am from England and very familiar with the area and airport.