eslHQ - Participle or infinitive?

Yes - although it's not always an enormous difference. I'll use other examples to make it clearer, and the verbs hear and [/i]smell [/i] as well, but the principle is the same.

Think about the difference between : A woman was screaming and A woman screamed:
a) A woman was screaming : the continuous form tells us that this is an on-going action - there were several screams. This relates to I heard a woman screaming.
b) A woman screamed : This time I understand a single, complete action - and this corresponds to I heard a woman scream

If you don't think of it as a choice between "infinitive and participle" but as a choice between simple and progressive aspect, it's clearer. Progressive aspect describes developing, on-going actions, and that's what's happening in (a). Simple aspect describes unchanging, complete actions - and that's what's happening in (b).

Some examples are fairly clear cut - if your dinner is cooking for example, it's clearly an on-going situation, so you'd probably say I can smell the dinner cooking. On the other hand, if a bomb explodes, it's a very sudden, quick event, so I heard the bomb explode. would be likely.

But imagine you look out of the window. A thief is breaking into the house opposite and, while you watch, he breaks the window, clears away the glass, and climbs through. And you rush off to call the police who arrive far too late to catch him, but come to take your statement.

Now - it took him a while to get into the house, and as you watched, for you it was an on-going situation (he was breaking into the house). So you could choose to present it like that : I saw the man breaking into the house.

But on the other hand, by the time the police get there the action is over - it's complete, finished. So you could also present it like that : I saw the man break into the house.

As always, the grammar just provides us with ways of expressing meaning. We then interpret the world in on way or other and choose the grammatical form which expresses the meaning that we want to give to the event.

In your example, you would probably see it as an on-going event : The children were playing in the park . And so I saw the children playing ... would be more likely. But given a context where you did want to present it as a complete action, then you'd choose the simple form.

Hope that helps.