London Heathrow to Kings Cross St Pancras with kids

Picadilly is one mode the whole way. Get on an empty tube with your luggage and kids. Get off at Kings Cross. Walk to St Pancras.

Heathrow express Get on as above. Get off at Paddington Station with your kids and luggage and wrangle everything to the Underground, then get on a (potentially busy) Metropolitan or Circle tube to Kings Cross, then get off with kids and luggage.

I've done LHR to Kings Cross/St Pancras many times. I take the Piccadilly line. You get on at LHR - there will be seats. Get off at Kings X quick walk to St Pancras. Simple.

HEX is great - if you are going to the Paddington Station area, but I, personally, don't see the advantages if your ultimate destination is Kings X. HEX is more comfortable and easier with luggage than the Piccadilly Tube and faster., but for me, that doesn't make up for transfer time and effort

I'm confused on your cost estimates. The cost would be similar only if you book HEX early enough to get a big discount. I don't know how your research could show similar costs if you are looking only a few weeks out, but I haven't explored what you might be looking at. I would expect HEX to be notably more expensive.

In addition to cost if that is an important factor for you, I wouldn't want to make the transfer from Paddington (using HEX) with kids and luggage. Others may feel differently.

I agree with the above posters. The Piccadilly Line would be the most direct and most cost effective.

What date are you traveling? There is work being done on the Piccadilly Line this year.

Piccadilly Lines Closures

Thanks for the advice and info!! Also thanks for the link for closure dates. Fortunately my travel dates avoid those closures.

I don't know about your kids but a lot of kids love looking out over the rooftops and seeing all the chimney pots (a la Dick van Dyke in Mary Poppins) as they approach central London from Heathrow. Most of the Piccadilly Line is elevated above ground except right at the airport and in Central London. There can be really interesting views. They can also follow their progress by checking off the stations on the map above the doors or on their phones if they have data.

I am personally not a fan of taking the Piccadilly line to/from Heathrow unless your destination is west London (nor as someone who lives here am I a fan of people taking up all the space on the tube with their luggage...). If it were me I would take Elizabeth Line to Farringdon and then go one stop on Circle/Hammersmith and City/Metropolitan line. That is about 10 mins faster than Piccadilly line and for me a more pleasant ride. The Heathrow Express will save you even more time (20 mins faster total journey time). If you get out at the back of the train the change to the tube isn't that bad, but I suppose it depends how much luggage you are carrying.

One thing to flag is if you are arriving at Terminal 4 I would not bother with the Heathrow Express option at all as you could be stuck waiting for a long time to transfer at T2/3.

Thank you all so much!
Are Kings Cross and St Pancras one station or are they two stations within walking distance of each other? Are things well demarcated with signage?

They are two main line stations across the street from each other. The tube station serves both stations and the signposting is very clear as to which exit to take for which station.

Within St Pancras there are three different stations, all clearly signed.
Arriving by tube the first of the three stations you come to is the Eurostar station, then Thameslink for cross London trains, and finally East Midland trains for long distance trains North

Within St Pancras there are three different stations, all clearly signed.
Arriving by tube the first of the three stations you come to is the Eurostar station, then Thameslink for cross London trains, and finally East Midland trains for long distance trains North

Actually 4.

Coming from the Tube, main corridor half way down on the right near Paul and other eateries is Eurostar and where other international trains may eventually start.

Towards the far end, on the left and downstairs is Thameslink to Brighton, Bedford, Luton and various London suburbs.

If instead of going downstairs you went upstairs on the left you have East Midlands Railway to Luton Airport and various towns and cities roughly following the M1 and A1 trunk roads.

But if you stay on the main floor and turn right before the Thameslink, just before you reach the doors to cross the street to Kings Cross are the escalators and lift up to Southeastern Railway's high speed Javelin commuter service to Kent and reaching as far a Dover.