OP, You need to be very, VERY, upfront here.
Yes, it sounds like this couple are depending on your kindness.
HOWEVER.
Not just one, but TWO, infant toddlers, more than once or twice a week is, very simply, not a 'kindness'.
That is a real job, a real responsibility, with real job expectations, and work, involved.
I would let them know that you want to clarify things on this child care.
And, to be honest... I am not getting the vibe from you that you really and truly want, or are 'up for' this job.
If that is the case, be objective and honest with yourself, as well as them.
I am thinking about the idea of 'let's do this a few times on a trial basis'.
These are young children, and I am assuming that the parents have jobs with some real work requirements.
I would want to be respectful of their needs to have more available and committed child-care set up for their young children.
I would not assume that it would be easy, or even possible, for them to find adequate child care for an infant and a young toddler right away.
They need to know NOW.
In fact, if they heard you say that 'Maybe, if we give it a few days and it works out...' the reason you have not heard back from them is because they are actively looking to set up other arrangements.
As a parent, I can tell you that when it would come to my young infant child/children, 'maybe' would not be good enough.