Oh well, since you've been driving since you were 17 then those years of experience will ensure you won't be distracted.
As for asking for a reply six minutes after you asked the question, I hope you're more patient when driving.
If you're preparing for a test, you should really be driving as near to test conditions as possible, surely a baby is a big distraction? I detect a big note of confidence about your test, due to the fact that you've been driving for some time. In my Dads experience, as a driving instructor, he said that, 'experienced' drivers were more likely to fail first time than new drivers. Beware!
There's also the fact that the supervising driver (the child's mother) should devote her full attention to supervising you at all times you are driving. What will she do if the baby starts crying or has some other problem and she's suddenly distracted from you? Another road user could die or be seriously injured as a result.
Another point. Travelling in a car is usually the most dangerous environment that any child will be at any time of the day. It's the one place they are in the greatest danger of being killed or seriously injured. (try telling that to those mothers who drive their children to school 'for safety reasons'!) Should this mother really subject her baby to such a danger as being in a car with you as an inexperienced and unqualified driver?