They're great in Primary. We (an International 4 form entry primary in the UAE) have a 1:1 scheme with Y3-6 and we have shared devices in Y1 and 2.
Being on Google Apps for Edu is a must - we assign each child an email on a separate domain (so if the school domain is myschool.com we have a 2nd domain www.myschoolapps.com) - in the format name-first letter surname-school year in which they started e.g. JohnS19
We have all student facing devices automatically fill out the myschoolsapps.com bit so the children just need to remember the first bit. As it is simple and tied to their name - we have suprisingly good success rate at teaching it to Year 1 children from the beginning of Year 1.
By October half term we have approx 50% of Year 1 able to login independently. 100% of Year 2 and up login day in and day out using their own logins.
It is worth perservering with in the early stages rather than use guest sessions
This means that children can use OAuth logins for a lot of sites, that have Google sign-in enabled, and even for sites that don't such as TTRockstars and SpellingShed, the password manager in Chrome remembers their login and auto-fills it.
We are Google everything now, and have achieved Reference School status.