An ICD is a device that could save your daughters life. I dont know the details of her condition, but if they are suggesting an ICD i assume its because they feel that she is at risk of having a dangerous heart rythmn that she might need to be shocked out of.
A lot of small children have pacemakers and ICDs for verying congenital conditions, maybe you should checkout some congenital heart disease websites so you can ask other parents how their child has fared with an ICD.
The main thing that you need to weigh up, and get on advice from her doctor are the risks of the operation, and what might happen to her if she doesnt have the procedure.
If it is really urgent and her veins really are too small to use (such as if she is a small baby) they sometimes open the chest for the first implantation, this obviously changes the type of surgery that she would be having. Ordinarily pacemakers and ICDs are implanted through a cardiac catheterisation, which is a reletively minor operation. I have had several such operations throughout my childhood and teenage years, although for different reasons than placing an icd.