Since no native speakers are rushing to answer, here's what the dictionary (Van Dale) says: a toddler or tot (literally as defined in the Dutch version: a child just old enough to start wearing pants).

From a bit of googling, I have the impression that by extension it also means cuddly (by nature or inclination) when applied to an adult, but you would do well to wait for input from native speakers in this regard.

Of course, a term such as this could have a derogatory meaning when applied to an adult. That can probably only be judged from the situation.

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Note added at 3 hrs (2007-11-16 14:35:42 GMT)
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'wee fellow' is another option

Een broekventje kan meerdere betekenissen hebben, afhankelijk van de context. Als een volwassene een broekventje wordt genoemd, kan het zijn dat hij zich kinderachtig (als een peuter of kleuter) gedraagt; het is ook vaak spottend bedoeld. Geef eens wat meer context... Wat had je gedaan of gezegd? Het kan ook gewoon plagend bedoeld zijn en is zeker geen sterke belediging.

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Note added at 15 hrs (2007-11-17 02:13:49 GMT)
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"Broekventje" has different meanings, depending on the context. If an adult is called "broekventje", he might have been acting as a child (a toddler); it often meant as a joke. Maybe you can provide us more context... What did you do or say? Maybe they were just teasing you. It definitely isn't a big insult or something like that...