It's about 2hrs 15mins from London St Pancras International with Eurostar. There are around 20 departures a day; under-5s travel free but don't get a seat if there’s not one available. When booking, do state if you are travelling with an infant so that you’ll be put in the family carriage with its baby-changing facilities. Children's prices are around £50 return; adults should look out for offers starting at around £69.
Eurostar arrives at the Gare du Nord. The taxi rank here can get very busy but if you're with young kids or need a people-carrier (ie, if there are more than 4 of you), make your way to the front and those in charge will prioritise you. Taxis are not absurdly expensive in Paris and are worth the outlay if you’re loaded down, but there are two RER lines and two Métro lines in the station itself.
There are flights from around the UK to Paris Charles de Gaulle just north of the city from most of its British airports and low-cost flights to 'Paris' Beauvais airport 1hr north of the city, in Picardy.
Depending on where you are staying, the RER (train) is probably your fastest means of getting into the centre from the main airports. A taxi from Charles de Gaulle will cost around €50 and one from Orly (south of the city) approximately €40.
In terms of getting around, the centre can be walked easily, and the Métro is clean and useful, if – like London's – a trial for those with buggies. The same tickets can be used on buses; to save money buy a carnet (book of 10) or a Paris Visite card, which gives you unlimited travel on local transport within certain zones for 1–5 days, plus discounted entry on certain attractions. You can buy it at all Métro and RER stops and in tourist offices, among other places.
Beware of Parisian drivers, who don’t always stop at red lights and almost never at marked crossings. The same goes for trying to drive here.
Adults and kids 14 and up can hire one of the bikes that form part of the Velib' system dotted all over Paris, although we can’t recommend strongly enough that you stick to cycle routes (see paris.fr for a map, including those in the Bois de Boulogne and Bois de Vincennes). A good walk (and picnic spot) is the Promenade Plantée, a disused railway line now planted with flowers, which runs from behind the Bastille Opéra 4.5km to the Bois de Vincennes.