I got the $2400 for me and my wife but not the $500 for my two children and they're under 17

https://www.irs.gov/coronavirus/economic-impact-payment-information-center

A19. If you did not receive the full amount to which you believe you are entitled, you will be able to claim the additional amount when you file your 2020 tax return.  This is particularly important for individuals who may be entitled to the additional $500 per qualifying child dependent payments.

Per the IRS:

If you did not receive the full amount to which you believe you are entitled, you will be able to claim the additional amount when you file your 2020 tax return.  This is particularly important for individuals who may be entitled to the additional $500 per qualifying child dependent payments. For VA and SSI recipients who don’t have a filing requirement and have a child, they need to use the Non-Filer tool (link) on IRS.gov by May 5 in order to have the $500 added automatically to their $1,200 Economic Impact payment. We encourage people to review our “How do I calculate my EIP Payment” question and answer (See question 11).

Economic Impact Payments