Can I get some info on the placement of alltid here? I know this is V2 kicking in so it gets moved to third place, but could the adverb come after the second verb? E.g. "Barnen vill komma här ofta"? Does it just depend on which verb it's modifying?
In English moving the adverb can change the meaning, in my example for example: "The children want to come here often" (They'd like to come more frequently) is different to "The children often want to come here". (They often ask to come here)
To begin with, it's Barnen vill komma hit ofta – since it's a question of movement, you need to use hit. The other one, här, is only for location, as in being somewhere.
You can get the same difference by moving the adverb in Swedish:
The children want to come here often" (They'd like to come more frequently) = Barnen vill komma hit ofta
The children often want to come here". (They often ask to come here) = Barnen vill ofta komma hit
One way to put this is to say that if you have two verbs together like here and the adverb modifies the first one of them, the adverb has to go after that verb and before the other one (otherwise how could you tell which verb it modifies?). So it has to be Barnen vill alltid leka because alltid modifies their wanting to play. If you wanted to say that they want to play "always", in this case you'd change the adverb rather than put it last. I don't know how you feel about 'The children want to play always' in English, but in Swedish, that word order doesn't really make sense. I'd say Barnen vill leka hela tiden ('all the time') or something like that instead.
There's a little more about the placement of adverbs in my general post about word order here https://www.duolingo.com/comment/8970470 but this is probably the trickiest area in Swedish word order.
I think word order is the hardest part of Swedish grammar for learners, and the placement of adverbs the hardest part of that part, so well… Also, any explanation about word order tends to become really long and complicated very quickly. So maybe your best bet is to try and get a general grasp of it, and then just try to listen and read as much as possible in Swedish so you'll get a feel for it. (but by all means ask questions whenever you're in doubt).