Childcare centres have family grouping in the mornings and afternoons when the numbers are lower - mine normally runs this until about 8am when more staff come on and the majority of children start arriving.
Centres must abide by the regulations regarding numbers in these groups, mixed aged grouping is 7:1 ratio, with limits on the numbers of certain ages in the group.
I haven't actually heard of centres who run this all day long, except under extreme circumstances, like in very quiet periods when numbers are very low.
Does this centre not have any other groups at all? What does the information pack say about their grouping? I cannot imagine having infants with 4 year olds all day long, very difficult to provide quality of care to all children involved.