Adoption is something about which you need to give very careful thought. You also need to bear in mind that all children who are awaiting adoptive families will have suffered from some kind of abusive past, physical, social, emotional, sexual etc and so will be affected bytheir past.
If you apply to adopt you will probably be invited on to a Preparation course where you will hear all about all aspects of adoption.
Re your own children - they will of course be affected by an adopted child joining the family just as they would be affected by another birth child, or indeed any change in the family circumstances. You also need to think how an adopted child willbe affected by/ fit in with your own children as they are of course a "little unit" which is not surprising.
i don't know the ages of your children but it is mostly likely that the adopted child will need to be the youngest and there will need to be a 2/3 year age gap between the adopted child and your own children.
Don't know if you are aware that most children awaiting adoption are older children (over 5 years) and sibling grous and those with disabilities.
Needless to say you and your DH need to be in this together and the children if they are old enough to understand. Having said that there is often one partner more ready to adopt than the other. You need to go slowly and think carefully about all aspects discussed at the Prep group and be honest with yourselves. Adoption is a risk and it is not for everyone. There is nothing wrong in "selecting yourselves out" of the process at anytime.
Also the adoption process is very lengthy and you will need to go through hours of interviews with a sw. to ensure your suitability to adopt. I'm not trying to put you off but it is something that needs much careful thought. Children awaiting adoption have already had a very bad deal in life and as far as is humanly possible it is important that they are placed in families who will be able to restore their trust in adults.
Please feel free to raise any more issues and I wil be happy to help if I can. I am an ex adoption social worker