Childcare

My daughter lives a 40 minute minimum drive from me and has a 10 month old son. She is back at work and asked me if I would come over to her house every Friday to babysit while she goes to the office for 4 hours. I work full time and adding on the travelling time each way she’s basically asking me to cut my working week by a day. Is this normal and am I being unreasonable saying no! I feel hugely guilty but I’m far from retired at 52!