Have you spoken to her father if he is of no help then
Keep your children at home, unfortunately for children there really are wicked (insecure) stepmothers out there. Whatever you have to do protect your children, from mean hateful people as well as influences that would cause a 5 year old to think she is going on 16. It may be cute now but when she is 10 and thinks she's grown it won't be. Encourage your children to be children, that's their job.
Find an older Christian woman who has been married many years and still loves her husband and has raised great children to mentor and help you.
Above all as the mother of a beautiful little girl and handsome little boy I strongly urge you to let them be the center of your life until they are up and college bound. Do not clutter their lives or put them at risk with any more stepfamily. You will find every sacrfice made worthwhile in the end. And with help you will find the journey joyful although trying.
God Bless, E.