I wish I could give you the helpful perspective that you are looking for, but I am not a parent. However, my father suffers from depression, as do I. As a child, I didn't know it by the name Depression. I just knew he didn't feel good.

Maybe, I could give you the child perspective...

I remember getting upset and frustrated that my Dad wouldn't pay attention or play with me sometimes. But then, he would explain it to me... in FEELINGS.

For example, I remember...

"Honey... I know you want to go to the park, but Daddy is tired and feeling sad right now. Will you help me? Work with me? I'll get you your coloring books and markers, you can watch a movie while I lay down for a while. I'll rest up, and tomorrow, IF the weather is good, we can go and play for a bit."

I didn't understand that he was DEPRESSED, but I understood he didn't feel well. I love my Dad. I know he loves me. He told me. He tried to show me... he did his best, and I did mine.

I wouldn't try to explain it to your child in any type of technical capacity. At least, not at such a young age. But, please, don't try to hide it. It will make things very difficult, and as your child gets older, it could hurt your relationship and trust.
Children are much more persceptive and intelligent (emotionally and intellectually) than we give them credit for.

As long as you try your best, you have nothing to feel guilty for. Your child DOES AND WILL understand that. To be your best, you need to put yourself first sometimes, and that is OKAY.

I hope this isn't a distasteful example, but...
If you're on a plane that starts to crash, you have to put your oxygen mask on first, (to ensure you stay conscious long enough) so you are able to help the person next to you.

No one who loves you, wants you to suffer ...or wants you at your "half-best" when all you need is some TLC.