Here's my gripe: I can find so-called "baby" raincoats from various online manufacturers, but upon closer inspection the sizing is intended for toddlers and not babies. Lucky for me, I am not one to give up easily. With persistence I've found 5 manufacturers who not only make baby rain coats, but make quality rain jackets with features like sealed seams and reinforced fabric.
My favorite basic rain jacket comes from tried-and-true manufacturer L.L. Bean, but I have got to say that the Tuffo Muddy Buddy (see link) is absolutely incredible at full-body coverage and is fabulous for little ones who like some serious mud play. Additionally, I've scoured the sites and also found a great baby rain parka if that is more what you are looking for.
The 5 selections of baby rain coats which I found are sure to please and several are sized for babies as young as 6 months old.