I tend to agree with noxlumos. . If reared sensibly a child of 15 should be capable of looking after itself for a couple of hours after school, and keeping an eye on a a younger sibling of 13 until you return from work. Make sure they know what to do in an emergency, and which neighbour to contact if there's a problem. Also tell them to stay in the house and not answer the door to strangers. I think kids are over-protected these days and need a little space to develop some independent skills, which includes looking after each other and learning to cope for short periods on their own. If you have set up appropriate emergency back-up arrangements, you will probably be surprised how well they cope. As to where are they supposed to go, your children should be old enough and have enough interests to stay at home and not roam the streets. Don't they have homework to do? Piano practice? Computers to work on? Books to read? Jigsaw puzzles to do? TV to watch? Model aircraft to make? Bedrooms to tidy? Giving them some free space may even help develop their self confidence. Perhaps it's time for the 15 year old to start learning to take some responsibility.
If you're still unhappy about taking this step, why not ask your employer if you can work part time?.