Universities certainly won't discriminate against you if you plan to take a gap year, especially if you have something constructive to do during it. Obviously, if you are applying for foreign languages, a gap year when you will be using the language would be a distinct advantage.
My advice to you on how to go about getting an au-pair job would be to look around for agencies specialising in the country or countries that interest you. Think about cultural differences and how easily you might adapt to them. As to what kind of locality you should go for, that depends on what you are used to, and what matters to you. You are supposed to have time off during the day to study, which would be easier to do in a city than anywhere else.
As for the ages of the children, you shouldn't be taking on children below local school age. For one thing, if you do, it may be difficult to get your proper time off unless there are good arrangements in place.
Being an au-pair can do wonders for your command of the language - and you will learn a great deal about another country's way of doing things, but do so in what should be a safe and friendly environment. Over the years - but not recently - I have come across several au-pairs and almost all of them went home enriched by the experience. Finding a friendly and considerate family is key.
An agency will advise on what you need by way of documentation and preparation. Take a look at
http://www.childint.co.uk/au_pair/abroad/europe as an example - there are plenty of others. Do be selective when picking your agency - they need you as much as you need them.