How Do You Explain "Step" Families in an Age Appropriate Way?

You just have to tell the basic truth, like you have been with your son. We are also a his, hers and ours blended family, and my step-daughter has half brothers on her mother's side too. The older kids never asked a lot of questions (possibly because they lived it, so they know their own stories) but my middle son, age 7, asks questions all the time and I just answer whatever he asks and no more. Today he asked me what my oldest son's father's name is and what he looks like. He asked how old my son was when his father left and when I told him that he wasn't even born yet, he was pretty surprised. Then he asked me how long I was married to daddy and I said 8 years so he said "oh. So you were by yourself with just C____ for 5 years?" and I said yes and he moved on to something else.

The kids will be piecing together the puzzle for a long time as they get older and can grasp things. When my younger kids ask me why their dad isn't their brother's dad or why I'm not their sister's mother, I just tell them that when my son was born, I didn't know daddy yet and when my SD was born, daddy didn't know me yet. As you know, it's never a once and done conversation. You just have to be prepared to get questions out of the blue at any time and answer the best you can.

I would just keep it very simple and answer literally what she asks. I wouldn't expand on things or add details, just listen to her question and what she's literally asking you, and then answer.

So she may ask you, "Why did Sam say that lady who picked up Rufus was his real mom?"
"Because that lady carried Rufus in her tummy before he was born. Do you understand?" Wait for her to say yes or no, then ask if she has more questions.
"Isn't Mrs. BradyBunch Rufus' mommy?"
"When Rufus lives in the house we visit with his brothers/sisters, Mrs. BradyBunch is like his mommy but he didn't grow in her tummy." Same thing, ask her if she understands, then ask her if she has more questions. She'll let you know when she's satisfied.

I have the same situation as you, and have had the same conversation.

You explained it fine to your son. If/when your daughter asks, just explain it truthfully and as simply and age-appropriate as possible. You had another husband, and he was your son's first dad. However, that is her brother, not her half-brother. Nit-picky labels like that are not necessary. You and your husband are mom and dad.

You might find she asks less than you think she will.

With your son, at this point I think you should wait until he asks questions, and once again answer them briefly but truthfully. You can finish by reiterating that your husband is his dad. A dad is the person who is there and loves you.

We joke that we are a venn diagram. We have his and hers and before their mom divorced, my stepkids also had a SF and stepsis and the stepsis has a stepdad and other siblings.

With DD I just say that her brother and sister have the same daddy but a different momma. It seems to work fine for her. And we have also talked about how Uncle S is her uncle because he married Aunt D so sometimes family becomes family by getting married.

I don't remember what my mom said when I was about 7 but she was more nervous than I was. She said that my dad wasn't my biological dad and I was like "Does this change anything? Can I go play?"

I think if you just give them simple facts and it isn't an issue FOR YOU, then it won't be for them, either. DD just accepts that sometimes SD is at school, or a friend's house or her momma's house.

I would just say he has a different daddy than you, but "name of husband" is like his daddy too. At that age, I don't think most kids care about their sibling not having the same dad unless we put that unintentional attachment to it (not saying you are, but some get worried and put that pressure on it without meaning to). Kids will learn as they grow. They don't need to know all the mechanics of it unless they ask somewhat specific questions about it. I wouldn't worry about it too much.