The mju II is an ideal size for a 5 year old's hands. It will also fit into his/her pocket as well. It is quite rugged and sealed to a reasonable extend against "the weather".
Fully auto gives instant success which is important for kids. If he/she loses interest then it is still a good carry around camera for use in instances where bigger cameras might be less manageable or less appropriate.
The plastic Kodak Brownie 127 looks like a "real camera" to a child and it is but it is a bit flimsy and maybe too big for a 5 year old's hands and with the demise of Efke are there any makers of 127 film left?
It also has a proper looking case which would fit around the childs shoulder so he/she can look like a proper photographer just like mum/dad.