Baby, or kids?
I have seen 18m outfits and 3t outfits from the same retailer that are almost the same size.
Baby clothes are annoying because you can't really try them on, so you may as well buy from a place that has easy returns.
And if there's a question of if it'll fit, buy a bit big and they'll wear them soon enough without immediately becoming landfill. You can do that in the store, you can't do that when you're buying online.
Better yet, buy used or get hand me downs from friends and family. Instead of sending even more stuff to the landfill, give things a second life. If a friend offers me a box of kids clothing, I'll rarely turn it down if its at least my kids sizes or larger. They'll fit eventually.
Once again, does sending most of what you buy immediately to a landfill seem sustainable to you? Does it seem good? If not, why are you supporting it?
Sure, my choices for clothing is more limited. But at least I've got the peace of mind I personally didn't send hundreds of articles of clothing to the landfill without ever actually wearing it. Its insane to me people seem perfectly fine with the idea of buying and throwing away so many clothes doing "returns".
The number of times I've bought clothes for my children and have it not fit after paying for it so far has been 0. I've messed up a few times on my own pants thinking the same size in the same pile would be roughly the same and been burned by it, but that's just led to me to force myself to always try before I buy that specific article of clothing.
But hey, feel free to keep throwing away 72% of what you buy thinking there's nothing that could be done. 72% failure rates are just what to expect in your life, true innovation being done by the sellers at Amazon.
Every time you click "return", think "send to landfill and get a refund". Think about how much junk you've just thrown away. Does this seem sustainable? Does it seem like a good thing to you?