travel on non us passport while waiting for US passport to be processed

Your child is a US citizen, so technically a US passport is needed (at least to re-enter, because when leaving, there is no passport control). If you intend to travel on the foreign passport and use the I-551 stamp to re-enter, it remains a fact that the child, a US citizen, cannot be denied entry. Proof of citizenship is essentially obvious without a passport for your child (your passport/naturalisation certificate and the child's birth certificate are basically most of the proof). But, as mentioned, we on the forum cannot advise you to do so, so I am just giving you my opinion to confirm/deny the opinions expressed.
 
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