Why would the bus driver discuss any issues of one student with another student rider, one age 10, at that? Certainly a strange process in bus supervision and problem reporting. 

Anyway, so what did the school say to your husband when they called/he called them to discuss the matter?  I assume the child's teacher/s was also confered with? How did the child's therapist recommend discussing this both in session and at home?

I don't think it's abnormal. I remember being 5 or 6 with a little boyfriend at school. I remember having crushes in 2nd grade. I was 8/9 when I discovered, uh, personal satisfaction. I didn't end up a teen parent or on drugs or anything.

I would just be honest with her and tell her she is too young to have those kinds of relationships with boys, but then tell her what an appropriate relationship looks like. Telling her boys are mean and to stay away is the surest way to get her to be all over boys, either to spite you or to find out what the big secret about them is.

Really, though, your DH needs to be having this conversation with his daughter. It shouldn't be focused on shaming, but on age-appropriateness. He can talk about how being friends, playing games together, and giving a quick hug is fine; however, holding hands and sitting in a lap is something she just isn't old enough to do yet, just like riding in a car without a booster seat. Letting her know that eventually there will be a day when holding hands is okay and relating it to something else she has to wait to do is the way I would go.

Depends on the child's background and history.

It is common behaviour in toddlers - 2  through 6 - to be gender or sexually aware or curious. It is part of their development for instance, to know that boys and girls are different. To understand their own physical biology - and the difference between "girls and boys". 

However this needs to be in an age appropriate context of their development  to be "normal" behaviour - as well as respectful of their own bodies and other people.  What is "normal" is for the behaviour to be transient, passing, out of curiosity and distractable. It can be a warning sign when it is not. And alarm bells should ring when it is coercive, aggressive or in some way indicative of adult sexual behaviour.

Your SD may just be more outwardly expressive in her interest."Boy-crazy" at a very young age and it may be a phase.  She may have been sexually abused whilst growing up or sexualised as a toddler without you knowing it. (In relation to the "more than friends" comment. Have you asked her what she means by that?) 
She may need the intervention of a therapist - and possibly the police.

My YSD informed us that she had 6 boyfriends when she was in maybe 1st or 2nd grade.  At first I was shocked... but regained my composure and decided to dig a little bit.  I asked her what did she think that a boyfriend was.. what did they do together.  She said "Well they are kind of like my servants.  They have to carry my books, bring me presents and if I want something off their lunch... they have to give it to me".  I laughed and said... well usually people just have one boyfriend at a time and that while i'm sure they are being nice.. that she should not take advantage of their generosity.

I would put little stock in the 2nd hand info from the older girl from the busdriver.. that IS odd that the busdriver didn't get a message to the parent directly or school if there was any concern.

If I were you, I would have her asked the question in an open ended way like I did... don't lead her or try to attach our adult notions of relationships to the schoolyard friendships.

Everything she is doing could be innocent, but her actions could indicate she has suffered some sort of sexual abuse. I would definitely talk to the therapist about it.

It sounds like SD is really a handful, but I can't help but feel sorry for her. She is living with people who are virtually strangers and I'm sure she feels like her life could completely change again with little warning. She may be acting out to "test" you to make sure you and DH are really going to be there forever. One of my neighbors takes introubled foster kids - if they don't make it with her the next stop is juvenile detention. She said most of them follow a pattern - their behavior is horrible at first - but once they figure out she is in it for the long haul and isn't going to give up on them they usually settle down.

So a few weeks ago, SS6 came home with marks from his teacher for being dishonest and when we asked him what that was about he admitted to kissing a boy on the cheek in the bathroom which the other student then ran and told on him. When he was questioned by the teacher her lied to her before admitting what happened. We asked him "why" because being in the industry I work in, we are always on high alert for sexual abuse in children. We asked about it SS innocently answered the boy is his bestfriend and he thought all bestfriends kiss because me and DH kiss, BM and Fiance3 kiss. All of this now falls on me and DH because ever since SS has started asking questions about BM and DH we have simply said  "BM and Fiance3 live together because they are bestfriends" "Dh and I are married because we are bestfriends" and the like. So we broke it down differently for him saying that kissing is for someone you want to marry. Do you want to marry this boy? (he said "no way! gross) so he needs to save that for when he is a grown up and leave his friends with high fives and hugs instead for now. 

Now with your instance with sitting in laps?...that's kind of alarming. I would start asking questions to her parent about who she sees do that so you know the context. Is it TV? Is it an inappropriate movie she caught a glimpse of or an adult asking her to sit on their lap...? I'd have many questions for that.