Paris Museum Pass and Metro/Bus Carnet
Note that all Europeans (EU nationals) under 26 get in free with ID, while
non-Europeans under 18 also get in free. If travelling with parents and they (parents) possess a museum pass or other means of fast entry then kids can enter at the same time.
Paris Museum Pass, to avoid queuing (lining up) for literally hours
this is a no-brainer, buy a Museum Pass from a tourist office or any one of over 60 participating museums (meaning most important Paris museums and some monuments). Passes last 2, 4 or 6 consecutive days and tourists can enter as many participating museums as they like, as often as they like (e.g. nip in just for the toilets!) without extra payment AND fast.
Here are some of the museums covered by this pass: Louvre; Army/Napoleon's Tomb; Pompidou; Branly; Arts; Art Decoratif; Conciergerie; Cluny; Moyen-Age; Musique; Orangerie; Orsay, Rodin and more.
Speed up use of trains or buses and reduce costs with a carnet
Taxis are very expensive. Most tourists will stay within Zones 1 and 2, the 20 arrondissements (districts) of Paris and will find a metro station within a very few hundred meters of anywhere. There are many complex systems to keep costs down when using the Metro, RER rail network and bus systems which cover the city thoroughly, but normal tourists spending a few days or even a couple of weeks will be well satisfied with a simple carnet (pron: 'carnay') of 10 tickets for 12 euros, as opposed to an individual ticket for 1.7 euros. Use them anytime.
More information on other multi-use transport passes.