i wouldnt worry about it too much. put on your cv or application form the date you left school to the date you would have started maternity leave 'various short term temporary contracts' .... and give any details you recall about what you were doing. i wouldnt think this would worry them too much as 15 years is such a long time ago. then add where you can that you have spent the last x years raising your children.
Like mandimoo says. Most employers are only interested in the past ten years or so in any case. I did once apply to a temp agency who wanted the fine detail of every tiny job I'd done since I left school, but I said I wasn't able - or prepared - to do that and I went elsewhere.
Try to concentrate on the skills and achievements of your years of being a mum.