Yes, the insole length of both sizes are different only by 1mm. Size 6 (C) has a bit more roomy fitting for sandals.
Baby Collection and Kids Collection are created independent of each other, to cater to different stages of your child's feet growth. Therefore, size 6 (C) cannot be treated as a subsequent size for 5 (C).
If your child has outgrown our 5 (C) in Baby Collection and you are looking to purchase in Kids Collection, it is recommended that you check the measurement once again before picking the size.