A Brazillian citizen holding a national passport do not require a visa to enter the UK for 6 months (I guess) for visiting purposes and 90 days for Switzerland.
Bear in mind that, Switzerland has one of the toughest immigration systems in Europe and their records can be second to none. Although your girlfriend seems to have lived there for a while does not mean they are not aware of her presence. I know of situations where some people in the last couple of years where mass deported.
It would be okay to travel to the UK (Immigration can refuse entry too) but as Dawie stated, returning to Switzerland can be a different issue upon re-entry and cross referencing of records. I would not take the risk. Are the children Swiss Nationals? Are they travelling together with you? This can help a little but no gurantees (I'm not an expert on Swiss Laws).
Check the information below for more reading:
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Who requires a visa?
Visas may not be required of citizens of the following countries, for stays up to 90 days:
(a) nationals of EU countries, Australia, Canada, Japan and US;
(b) nationals of countries in South and Central America (except nationals of Belize, Bolivia, Colombia, Ecuador and Peru who do need a visa);
(c) nationals of Caribbean island states (except nationals of Cuba, Dominican Republic and Haiti who do need a visa);
(d) nationals of Andorra, Brunei, Croatia, Cyprus, Czech Republic, Estonia, Fiji, Hungary, Iceland, Israel, Kiribati, Korea (Rep. Of), Latvia, Liechtenstein, Lithuania, Malaysia, Malta, Monaco, New Zealand, Norway, Poland, San Marino, Singapore, Slovak Republic, Slovenia, Solomon Islands, South Africa, Tuvalu and Vatican City;
(e) nationals of Bosnia-Herzegovina, Former Yugoslav Republic of Macedonia, Turkey and Serbia & Montenegro provided they are resident in Canada, Iceland, Norway, USA or EU countries.
Nationals of countries not appearing above, or those staying longer than 90 days may require a visa to study in Switzerland.
Before leaving for Switzerland, you must complete certain important formalities which you should begin as soon as you receive confirmation that you have been accepted by a Swiss university.
http://www.learn4good.com/travel/swz_visa.htm
Official Document Link:
http://www.eda.admin.ch/etc/medialib/do ... _Reg_e.pdf
http://www.weisungen.bfm.admin.ch/einre ... iste_e.pdf
Obligation to hold a visa
http://www.weisungen.bfm.admin.ch/einre ... rschriften
