Vitamin C supplements help with the absorption of non-heme iron. This has been known for some time. Give them vitamin C supplements along with their ferrous sulphate.
Vitamin C also helps with absorption of heme-based iron supplements such as ProFerrin ... even THAT stuff was constipating me until i loaded up with Vitamin C along with the ProFerrin ...
Vitamin C much less expensive than Guava juice and zero sugar ...