I think the best comparison to the Against Malaria Foundation would be the Nurse Family Partnership (NFP), which also primarily benefits young children (albeit through a different intervention). Olds et al. (2014) reports the results of an RCT examining the effect of NFP on child mortality and maternal mortality,[1] and GiveWell has an (old) estimate of the cost per child served for the Nurse Family Partnership.[2] You could potentially use these two sources to arrive at a rough estimate of the cost per death averted for NFP.
I think the best comparison to the Against Malaria Foundation would be the Nurse Family Partnership (NFP), which also primarily benefits young children (albeit through a different intervention). Olds et al. (2014) reports the results of an RCT examining the effect of NFP on child mortality and maternal mortality,[1] and GiveWell has an (old) estimate of the cost per child served for the Nurse Family Partnership.[2] You could potentially use these two sources to arrive at a rough estimate of the cost per death averted for NFP.
[1] https://jamanetwork.com/journals/jamapediatrics/fullarticle/1886653
[2] http://www.givewell.org/united-states/charities/nfp