For babies that are still at the lying stage I prefer to do with shorts, a nappy cover, or with white body vest on, as I find clothes do not fit very well for this age group.
For older babies and children then solid plain colours are best for photographs and something that fits well. Avoid collars if possible.
Do bring a change of outfit, a hat or any other item that is unique to your child – their favourite teddy or toy for example.