Your kids are 4 & 5 years old. Perfect age for this, and your new role. AND they are old enough for simple chores.
You explain to them... that Mommy has a job at home. I work from home too. My kids know this. During their naps, that is when I work. Or at night after they go to bed.
I also tell my girl, who is just making 6 yrs. old, that anything she can do herself, she can do by herself, since she is her age. ie: "instead of asking Mommy all the time to get you a cup of water... you can do it yourself..." or, "if you need a crayon... you know where to find it and you can get it instead of asking Mommy...." All these 2 second requests every 5 seconds by my daughter really was nerve-racking. So now, my girl has realized, that Mommy is there, but if she can do something herself, she CAN do it. Like my Hubby said "Mommy is maybe too nice.... she does everything... and you need to help too..."
For me, what also helps.. .is that we have one room in our house which is a "quiet room." And in this room, they can do anything they want, but it must be "quiet" and not boisterous, and no "fighting/yelling." They understand. This is the room we go in before bed or nap time. This is where they wind-down and deflate. It HELPS to organize yourself and the kids, and keep sanity.
We also have another room, ie: the "play-room" in which they can do anything, noise and all, and even make a mess, and I don't care if this room is messy, because it's the "play-room." It's fine. My kids understand this. And it helps ME too, because if they are in the play-room, I don't mind all the noise they make... it is okay and it is their "noise" room.
So, having these 2 "rooms" with it's own "roles" have helped me to organize the kids and myself. It serves a purpose, and the kids know it. They even will tell Grandma "we're going downstairs... that's the quiet place." Or, "we don't make noise in this room...." etc.
When I am "working" my space is right near by... but the kids pretty much "know" what they can and cannot do in the "rooms" and that I need quiet when I am at the computer working. Once I say I have to "work" they know what it means. So, that is why I say, at your kids ages... they CAN certainly understand, per your explaining to them... what is necessary. Sure, they are just kids... but they do learn. My girl actually told me she is "proud" of me that I work at home and can still be with them. I even made a sign, saying "Mommy working...." so that my girl will know visually, when Mommy is busy with something, that can't be avoided.
On a daily basis, I have a "schedule" for everything.... my kids nap at the same times everyday, (and they know it), they know their and my "routine" and so they know what is coming up without my having to "nag" about it. They know when I cook dinner, when they go to bed, when is snack time, when is play time, when is just "whatever" time. There is a "framework" of activities everyday... and since I do it EVERY day, the same... a child WILL learn the routine as well... and then be able to "anticipate" what is next, what they must do, and so forth.
I also put on index cards on the wall, my daughter's "chores" or what she needs to do everyday. Even for brushing her teeth. It's just a "reminder" for her. Then I don't have to verbally "nag" about it everyday.
No I am not "anal" and rigid about it, but I do keep to a routine... and it REALLY helps me to modulate myself, and the kids. Kids learn by repetition and constancy...and routine. Once they get used to it, and the routine, it really helps in their daily life too.
Just some tips I have done. There are SO many.
Hope this helps,
Susan