Are casein proteins in UHT milk digestable for infants or toddlers?
UHT treatment increases the digestibility of casein.

Ultra High Temperature Treatment, but Not Pasteurization, Affects the Postprandial Kinetics of Milk Proteins in Humans (Journal of Nutrition)


  Moreover, the UHT treatment of milk causes structural changes in its
  protein system. The main change is the denaturation of whey proteins
  and their interaction with casein micelles. Consequently, it was
  expected that the behavior of the UHT milk would be similar to that of
  casein micelles, as observed with native casein. However, it has been
  shown that the interaction between casein micelles and whey proteins
  is also accompanied by a large increase in number of very small,
  soluble particles resulting from the desegregation of casein micelles.
  Consequently, by weakening the interaction forces between caseins,
  heat treatment provokes both a loss of the micelle framework and the
  formation of small aggregates. Correlatively, an increase in the
  amount of nonsedimentable casein has been observed in UHT milk
  following high-speed centrifugation (33). Two effects of these changes
  on milk properties may be softer coagulation and a higher
  susceptibility to enzymatic hydrolysis due to loosening of the
  micellar structure and the presence of small aggregates that may be
  more accessible to enzymes.


In the stomach of adult or infant, gastric acid and the enzyme pepsin denaturate whey and casein proteins from either raw or heated milk Critical Reviews in Food Science and Nutrition). The denatured proteins are then nearly completely digested by the enzymes pepsin in the stomach and by trypsin, chymotrypsin and peptidases in the small intestine (Medical Libretexts.

Humans, including infants do not have the enzyme rennin (chymosin).


  Rennin, also called chymosin, protein-digesting enzyme that curdles milk by transforming caseinogen into insoluble casein; it is
  found only in the fourth stomach of cud-chewing animals, such as cows.
  Its action extends the period in which milk is retained in the stomach
  of the young animal. In animals that lack rennin, milk is coagulated
  by the action of pepsin as is the case in humans. A commercial form of
  rennin, rennet, is used in manufacturing cheese and preparing
  junket (Britannica).
  
  Chymosin is secreted in the neonatal stomach of ruminants (cattle,
  goats, camels), pigs, cats, and rats. Animals including humans,
  chimps, and horses have inactivating mutations in their chymosin gene
  and do not secrete the enzyme (vivo.colostate.edu).


There's one study that mentions the presence of chymosin in humans, but the evidence seems to be very weak as described in another Biology SE answer. Some articles, like this one mention chymosin in humans without proper references.