I'm with Crystal that a happy mum is a good mum. You only work 3 days a week and so don't feel guilty. It's important that you think of your own future so you have a life after your children have flown the nest.
I'm also on maternity leave after having my second child and I still send my eldest to day nursery 3 days a week. Not only did I want to lose his place it would maintain some routine for him and allow me one-on-one time with the baby. I will be returning to work fulltime and my mum will resume looking after him the other 2 days. I know that he's enjoying the extra time with me and will be resentful when I return to work.
I would tell a white lie to your son and explain that you have to go to work so that you can buy him nice things. You'll just have to ensure that you spend quality time with him as well as his sibling.

I must remember to heed my own advice when I go back as I'm sure I will also feel guilty of being a bad mother. Like you I couldn't be a fulltime housewife and my mum worked fulltime and I don't think I'be been psychologically harmed as a result!

Thank you so much for your advice I really appreciate it, its not easy being a Mum at all (hardest job I've ever had!) but I guess you can only do your best. My Mum also worked full time when I was a child and I felt I benefited from having someone else there like a friend for me.

I think the mistake I made was having him at home with me when baby was born, although he went to playgroup twice a week in the morning but I think the change in his routine probably didn't help. Its also made our daughter very dependant on him, she'll be lost when he goes to School!

Thanks again and good luck Amanda when you go back to work.