Wow there an awful lot of assumptions on this thread!
The childminder looked after the child for 3.5 days and presumably, since the op hasn't said she was unhappy at collection time, has tried to settle the child without making a big deal of it to the parent for at least two of those days before it became too much for them all?
A 3 year old screaming at the baby to stop crying isn't nice but clearly just shows how consistent the crying had been, and the 3 year old was, well just being a 3 year old. The fact that the childminder used this incident to portray how bad the crying had been says a lot because clearly it would have been more beneficial to her to not mention it at all.
The children I mind come term time only because there are cheaper alternatives available to the parents at holiday clubs etc. However, I charge a retainer during the holidays (which the parents are welcome to use but mostly don't) to protect their place for the new term. The retainer is non-negotiable and non-refundable regardless of whether it's used or not.
I have been minding for 17 years. I have had two children that I have had to terminate the contracts for in all that time. The first was a 4 year old who would not move from the front door and screamed the house down for 4 days solid whilst trying to escape - I terminated on the 5th day but kept the 4 days money that he had been with me. The other was a boy of 7 who I terminated after a term because he disrupted the house so badly that it was unfair on the other children - if the parent had been honest about his behaviour in the first place we may, or may not, have been better prepared and able to make it work. I have been in childcare for 40 years ands it's not a good feeling admitting that these two children were just not right for my home.
And anyone who thinks there aren't difficult children, or spats, or kicking, biting, screaming, and falling out at some point in every form of childcare setting is delusional.