Hi Paulie,
As long as the children and the other parent agree, there should not be a problem. If, due to some unforeseen circumstance such as one parent falling ill, you cannot fulfil the consent order or agreement previously agreed upon, as long as you made reasonable efforts to fulfil it, it is okay to deviate from the agreement.
This is more so the case where you do not have any consent orders or custody agreements in place.
I would suggest writing this temporary arrangement down in writing, dated and signed. In brief, it should be perfectly fine to have one parent look after the children, contrary to agreement, if the other parent is not able to.