We haven't simplified their login at all but what we have done is ensured that the devices they are using remember the login details inclusive of passwords. This obviously is not great in a shared device environment but in the 1:1 that we have it works a treat!
This has been a struggle for our teachers too. I do know that some take the time to go around and login the younger students themselves. Maybe not too productive but once in the students know to navigate.
Have you tried having the younger students handwrite it out? I use to have them handwrite their login information on all written assignments (maybe better for 1/2). It really helped build their memory of it!
For kinders - I have put stickers on the keys that they needed to log in. I used number stickers so they just had to follow number order. That gets a little tricky if you have multiple kiddos using the same keyboard.
Another great program that worked really well was at one K-6 school I taught at. I created a technology "buddy" system. So, a rotating group of 6th graders would come and help login the kinders, a rotating group of 5th graders would help login the 1st graders, etc. The goal was not for the older kids to do it for them, but to scaffold the learning and get the younger kids comfortable with logging in.
The good news, in 12 years I have always, always seen these younger kiddos master the login process, it takes some time and consistency, though.
In the past, I've seen this handled with a set of index cards (one per student) with their login information on the card so they can see exactly what to type in where. A printed screenshot with their login information superimposed over the fields where they should type it would be even better. I'm not sure if modern browsers have an easy way to make it not obscure the password field - I feel like at some point in my life I had a way to do this but couldn't find one in a few minutes of halfhearted poking around in Chrome. If such an option could be found, it would probably be helpful for kinders trying to slowly match letters and make sure they typed the right ones.