Madusa - there is an urgent need for foster carers, though for all aged children, not just babies. Your children are a good age to think of fostering. You really need to as Flower says to contact your social services dept and tak it from there. You do need to know however that there is a lot more to fostering these days than just looking after babies. There has to be contact with the birth parents, social workers and all sorts of others involved in the case.
You need to get as much info as you can from SSD and then if you are still interested you will be invited on to a Preparation course and learn about all aspects of fostering. If at any stage in the process you feel it is not for you then you can pull out without any committment. Incidentally your H/P has to be as motivated to foster as you are for obvious reasons.
After the prep course a social worker will do an in depth assessment of you and your H/P to consider your suitability to foster. You don't need to be perfect of course (as on one is) but you do need to be non judgemental, flexible, child centred, able to work as part of a team and a sense of humour helps. finally your case goes before a fostering panel for final approval.
People change their "offer" over time and many who start off only wanting babies will consider older children. However with your youngest at 5 there would need to be a 2/3 year age gap so at the moment you would probably be best thinking of 0 -2. You may not get as many placements with this age as there is moe need for older children but as your children grow, you may change your offer.
As for your children, yes they will be affected but not adversely necessarily. You will also get the opportunity to talk with experienced foster carers who I am sure can put your mind at rest about the effect on birth children. Yours are at a good age to be thinking of fostering, neither too young, nor teenagers who might not want young children messingup their stuff.
Good luck and be happy to help further if I can. I am a retired social worker with over 30 years experience in the field of fostering and adoption