You won't win any popularity contests using the term 'SEN boy'. If I was you I'd ask MNHQ to change the title to something about 'boy used sexual language towards DD in school'. Then ask them to edit the OP using the terms a 'boy who has SEN'.
I'm not going to ignore your post though because as a mum of a child with SN whom themselves has been the victim of bullying and sexual remarks I understand what your going through.
I sent an email to the HT. I made sure I was 100% sure of what my DS involvement and actions had been with regards this incident (quite easy as my DS doesn't know how to lie!) and emailed the HT. Make sure the email is short, concise and unemotional. Make you you use words such as well being and safeguarding. Don't give small facts just a fact of 'what was said' and ask for HT to investigate - the more you give them the more you give them things to defend rather than answer and/or deal with the actual issue in hand.
Check out their behaviour policy and bullying policy and only quote a passage from here of necessary.
For example:
Dear HT,
I am concerned for the well being of DD due to recent incidents in school.
*on x date at y time Z said "......"
I had an off the record discussion with CT in X day at y time.
Further incidents have occurred and therefore I am concerned that safeguarding efforts are not effective at this stage.
I look forward to your comments when you've had time to investigate.
Yours ......
Do please bare in mind that if you DD repeats what is said then she will also occur a punishment. However politely point out that although it's misguided use of words she knows is wrong it's not done in order to cause upset or fear. It's best not to be defensive and act like your child is gods given angel as it gets you know where.
The HT was an absolute dickwad to me when I called school up on safeguarding after 2 pupils threatened to rape DS and told him I'd rape him when we got home.
Comments and replies were as follows....
HT"We do not believe there is a safeguarding issue and that your DS is safe"
ME " recent comments are evidence of the contry"
HT " we don't believe this is after repeated sessions where these boys have bullied DS" (ref previous complaints from DS and I about these lads)
ME "you have previous complaints which are logged as I always email.
HT" well we know everything that's said in class so we would have picked it up" (with ref to this being isolated and not end of long term bullying)
ME" then you would have known this threat was made so I'd like an explanation why you ignored it originally"
HT" well these are generally nice boys"
ME " are you saying my DS isn't?"
HT " these boys say your DS is annoying to them too"
ME " no doubt my DS is annoying to his peers. He has a social communication disorder - however a disability is not bullying. Your bullying policy states bullying is.....intent to cause harm"
I was also told my DS needs to learn to tolerate a 'bit of banter'. I highlighted bullying is not banter and I don't believe this incident was banter but as we couldn't agree as all involved are 10 yo we should contact police and SS for advice.
The poor man nearly fainted 
Don't let people minimise this. If the child has SEN and cannot distinguish between right and wrong, appropriate and inappropriate the school needs to support him. Perusing this correctly and calmly will get all children involved in this the right support.
Sorry for war and peace!