Can 'too'be used immediately after the subject?
Both of your sentences are grammatical. However, most style guides would recommend putting the too in your first sentence between a pair of commas.
For instance, The Chicago Manual of Style (17th ed.), 6.52:
When too comes in the middle of the sentence or clause, however, a comma aids comprehension.
She, too, decided against the early showing.
Following that guideline, your sentence would look like this:
Let her, too, wake up to a hot coffee.
But that's a matter of style. It's still grammatical without the commas.