Deductions & credits

‼️EDIT‼️
My younger children is 9 months (turns 1 in 2021). My other is 4 years ( turns 5 in 2021) .  My other children are 9 and 12 years old . 

I know I’m receiving $800 because I checked the IRS site. I’m still short $300. I’m assuming it’s because they didn’t put our 9 month old who was born in October of 2020.