Language Arts includes several aspects: literature, composition (written & oral narration), copywork, dictation, poetry, elocution, and grammar.
For your first grader, there isn’t really anything you should be doing except oral narration, copywork, poetry, and literature.
For your third grader, you can do aspects of all these, except probably formal grammar (such as PLL or JAG). Charlotte didn’t recommend beginning formal grammar instruction until age 10, and I absolutely agree w/ her. Grammar is a finite body of knowledge that is never going to change. Why should children learn it in first, second, third, fourth, fifth grades and so on? A noun will always be a noun, a verb will always be a verb, forming contractions, thinking of synonyms, etc. can come much later when the child is at that level of thinking and understanding. I’m just touching the tip of the iceberg w/ my year four 9yo dd with English for the Thoughtful Child.
It sounds like you’re putting a lot of unnecessary pressure on yourself. If both of your children are good readers and can give good narrations, you’re doing a great job. Touch on poetry once a week. SCM has three fantastic new poetry resources that make it pretty idiot-proof. 😉 When my children were in years 2 and 3, I added in once weekly creative writing. They draw a topic from a jar of ideas I’ve already put together and write about it. I don’t check punctuation, spelling, or grammar. It’s simply a way for them to practice putting their thoughts together. Amazingly, correct grammar, punctuation, and spelling happen more and more over time. Maybe introduce prepared dictation using SCM’s Spelling Wisdom next year when your dd is in fourth grade.
I would not be overly concerned w/ writing skills at this point. Honestly, my children just get better and better because I don’t force programs and practice on them. It comes gently in the form of oral and written narrations, creative writing, and reading lots of good books. Children catch on to these concepts naturally the more they’re exposed to them.
As far as short lessons go, your first grader shouldn’t be doing any 15-minute lessons, except for maybe math. That’s the maximum time for lessons at that age. 20-30 minutes is plenty for your third grader, but other than reading independently for that time, there’s no reason why dictation, poetry, narration, copywork, or even introductory grammar should take that long. If you feel you need to be doing something daily, how about this:
Monday: copywork, read aloud w/ oral narration (this can be history, literature, or a book just for fun)
Tuesday: copywork, read aloud w/ oral narration, poetry
Wednesday: copywork, read aloud w/ oral narration
Thursday: read aloud w/ oral narration, creative writing
Friday: copywork, introductory grammar such as English for the Thoughful Child orally, read aloud w/ oral narration
That’s covering language arts every day in several different subjects, but shouldn’t take too long and definitely wouldn’t be complicated.
Even for my older children, we do a max of 5 minutes of best-effort copywork 3-4 days per week. Dictation is twice per week and takes no longer than 10-15 minutes from start of study to completion of exercise, creative writing takes them as long as needed to complete the topic giving their best effort, oral grammar exercises w/ dd take 2-5 minutes twice per week, reading poetry aloud takes no more than 5 minutes daily. It’s very simple and straightforward.
Hope that helps. Please ease up on yourself and your children. I speak from a place of experience. I tried to push too much too soon, and paid the price for it later. I’ve learned my lesson the hard way and am a less-is-more/later-is-better advocate. You and they will burn out if you’re trying to do all this right now. You’ve still got 9 years left with one and 11 years left with the other one. Plenty of time for lots of language arts! 
Blessings,
Lindsey