If your child is going to be a flower girl or page boy at a wedding then the bride and groom may specify what outfit they want them to wear. If the couple has bridesmaids of all different ages then they may struggle to get outfits that are available in all sizes to allow them to match. If this is the case then it is often better to go with a colour that you want rather than an exact style as you will often find this easier to match.
If you are wedding guests then you may want your child to be quite dressed up but before you spend a fortune on an outfit for them, think about if they are going to be able to wear it again afterwards. Children at parties often spill food or drink down them, and crawling around on their knees or playing outside so it is not ideal for them to be wearing light colours that easily mark. If your child is still quite young, then they can very quickly have a growth spurt meaning clothing that fitted them a few months ago no longer does. Why not look out for bargains in charity shops or see if you can pick something up in the sale if you want to try and save a few quid.