One unhappy mother needing some advice here! 

My eldest daughter is 7 and one of her friends has a phone, (this was actually when they were 6) so has swapped numbers with my phone and I’ve said I’m completely happy for them to message/call my daughter when we aren’t busy etc so that they can talk! 

My child previously has mentioned about her friend having YouTube and a phone which personally I think it’s wrong having socials at that young age and I’ve made my feelings very clear about this in front of the child’s parents, also telling my daughter she has no need for a phone at the age of 7.

Now this child I’ve had to have words with about before as if she’s been over a few times to our house for play dates (with her brother) who mother guilts into coming over as it’s supposed to be her daughter only but last minute knowing I won’t say no she has gone oh can \*\* come too, he really wants to..   
anyway they’ve done some terrible and dangerous things that even my 3 children have been horrified with and have told me straight away because they know better.. and mine aren’t angels is all I’m saying! 

well this just puts the cherry on top.. 

This morning I’ve received this from the 7 year old, message meant for my 7 year old.   
I’ve gone to message the parent but I’ve had to take a moment because I am absolutely raging and also wondering what her children are seeing and getting up to at home like WTFFFFFF!!   
I was going to say I’m lost for words, but I’m not, in fact I have many words that I’m trying to hold back on saying to these parents right now. 

Also what with the previous things that they have been at my house and done or said (the girl is 7, the boy 5)   
Am I right in reporting this to the school? Last time they came to our house they were told it would be the last time, and mark my words it has been, only recently have I been thinking of allowing the daughter round in the summer holidays to come out with us etc, this will no longer be an option. 

Should I just crack on with my message to let her know I am absolutely disgusted and fuming and that she needs to be keeping an eye on her children as I’m sorry but I think this is unacceptable or should I try approach this situation a little nicer? I’m not saying I won’t make my feelings clear? To be honest I can’t see any other way to approach this I’m so angry!

How would you approach this situation as a parent? Absolutely FUMING!!