Each pupil at my school logs into the school computers using the same username and password. It's a system I inherited and when given the option a few years ago to give each pupil their own individual login, I opted not to and instead to just carry on using the same system. Whilst you may think that this choice isn't the best security-wise normally, I would argue that it actually is in the primary school environment. For starters, this generic pupil account is very restricted - all settings changed when logged in aren't saved when logged off. It also has a high level of Internet filtering and children can only 'write' new data into their personal folder - they can't 'delete' anything. This means that I can be safe in the knowledge that children can't actually cause any damage - either accidentally or maliciously. All computer usage is monitored in some way by an adult too - children can't use machines without seeking permission first so they are always aware that a member of staff will be around to check that they are using them properly/help them if needed. It also means that staff can set machines up beforehand to save time occassionally (e.g. laptops in a classroom if they fear an assembly might be longer than usual) and children can take turns at working on machines without having to waste 3 minutes logging out and back in again, wasting valuable learning time.
With online accounts though, I take a different stand and every child has their own unique login. This is because they might be accessing the service at home (when usage could be monitored less) and because children can't access different folders when logged in (i.e. they have their own user area which can only be saved into when logged in as that user). Over the years I've given out a variety of username formats, but I now think that I've happened upon the best - ForenameXX (the child's first name followed by a two-digit random number). This includes: a capital letter, some small letters and some numbers so is crucially a mixture of characters. Upon request of a member of KS1 staff, it also includes their forename rather than their surname as younger children find this easier (and therefore quicker) to type.
Our children have access to three online services that they login to - Edmodo, Google Docs and Purple Mash. The latter two allow me to set usernames how I like whereas I must choose a username not already taken globally on Edmodo. For this reason, I just create a user account up on Edmodo first and then use the same one for both Google Docs and Purple Mash.
For a password, I initially set all accounts to login using the same password and then prompt the children to change this when they first login - this helps them take a bit of ownership for it.
Once all accounts have been set up in reception, in theory they should then just follow the child up through the school and don't need to be changed at the end of every academic year. All our online services, together with the school website, can be accessed from www.parkfieldict.co.uk which the children find quite easy to remember when they want to use them at home. This is also set as the home page in school.
I maintain a spreadsheet containing class lists in different tabs, with each child's username alongside their name - I've found this is much easier to refer to quickly in a lesson than logging into a service's user management area and it can also be printed out so I can carry it around with me at the start of a lesson when children are logging in (to save me having to keep walking back to my computer at the front).