Passports, visas and the associated red tape are all an inevitable part of modern international travel. The first point worth making here is that there are no passport checks or border restrictions of any sort when travelling between England and Scotland. From this point of view, the UK as a whole operates as a single state, so passports and visas are checked at your first point of entry into the UK, and not when travelling within the UK.
In order to gain entry to the UK, all foreign citizens need a valid passport, and children will need to be covered either by inclusion on an adult's passport or have a passport of their own. Once here, citizens of European Union countries may stay as long as they want. Visitors from the USA, Canada, Australia and New Zealand can stay up to six months, as long as they can show evidence of a ticket home and the funds necessary to cover their intended stay. Citizens of many non-EU European countries can stay up to three months. Visitors from anywhere else will need a visa from the British Embassy or Consulate in the country they are travelling from: the official FCO website gives access to definitive advice about obtaining a visa to travel to the UK.
Visitors intending to work in the UK, including in Scotland, who are not citizens of a European Union country, will need a work permit to do so legally. Again, the FCO website gives definitive current information.