Firstly, what do your children want to do? Their wishes and feelings will be taken in to account, but depending on their ages, may not be paramount.
I presume that you don't want them back with you, but feel they would be better staying where they are?
Did the court order stipulate what the statement is supposed to cover? If not, then I'd suggest you simply set out what you believe to be in the children's best interest.
Sorry I can't tell you what to put in the statement as I'm not sure what your position is, but in general terms, the statement should be set out with the title of the case at the top, inc. case number in the top right hand corner. Start off my saying that it is your first statement, made in response to the order of Judge x made on x date. Then use numbered paragraphs setting out a brief historical background, and your point of view as to what is in the children's best interest, why you think so, and how you think that can be achieved.
Sign the statement, date it, and file one copy with the court and one with your husband's solicitors. Resist the urge to rant, or to state anything you cant back up in court under oath. The statement will be referred to at the hearing and anything weak or untrue will be subject to being contested by your husband's solicitor or barrister.
Read it over several times before you sign and serve it. You may want to think about getting a Mackenzie friend to help you, and to check your statement. They can't tell you what to put in but can advise if the statement is technically messy/incorrectly set out/legally weak.