Firstly, the practice of marrying locals in the crusader states was not uncommon, but it is very unlikely that anyone arriving to the East on a temporary Crusade, rather than to stay for an extended period of time, would have had legitimate children (that would have survived in the historical record) by local women. I'll answer with reference to those who settled in the East, and their children - hopefully it will be of some interest.

Even the very high nobility of the crusader states married locals. Baldwin II, the king of Jerusalem, wed the Armenian Morfia of Melitene, and their children intermarried into the local nobility and even into the Byzantine royal family, which also supplied brides and grooms to the high nobility of the crusader states. William of Tyre, bishop of Tyre and a historian resident in Jerusalem, wrote in the 1120s on assimilation with locals, stating that some Latin Europeans had married Syrians, Armenians, or even baptised Saracens. This suggests that it was commonly done.

(Regarding previous comments on race - the locals were seen to possess a separate identity from Latin Europeans, which I think is relevant here. Although race as such was not particularly important, there was definite interest on the various groups in the area, that were separated by ethnicity, culture, and religion).

The phenomenon was also common enough, with other markers of assimilation, to prompt comment from many European observers and historians, who denounced the Syrian-born Latin Christians as "pullani" who had adopted Muslim customs. This is a frequent feature of Western histories, with Jacques de Vitry offering an especially scathing critique, and he blames them for Christians' difficulties in the East: they "were brought up in luxury, soft and effeminate, more used to baths than battles". This alone may have acted as a disincentive for returning.

As for returning to Europe, the issue is a bit murkier. I went through some family trees of the rulers of the crusader states, and cannot seem to find any who returned to Europe en masse.

In the East, there was a chronic lack of manpower, with readily available land and ongoing warfare, so issues that would normally drive sons to migrate, lack of land and the search for prestigious military opportunities, were not as prevalent. The infant mortality was high, as you would expect at the time, especially in an area with diseases to which Western European medicine was unaccustomed. Often, one only finds one or no male heirs for most generations, and these heirs had no incentive to abandon their inheritance.

For daughters, there was much intermarriage within the Eastern Latin kingdoms, and the same lack of manpower was apparent here: marriage alliances for the queens regnant of Jerusalem had to be sought from the West to gain appropriately illustrious matches. Some brides went West: Philippa of Champagne, the daughter of Queen Isabella I of Jerusalem, married a French nobleman who encouraged her to press a claim on the county of Champagne. Similarly, Agnes of Antioch married the future King of Hungary Bela III. It thus seems to have been primarily women who moved from the Latin East back to the West, and unfortunately not very much is known about their experiences.

One finds some more people returning at the end of the 13th century, coinciding with a strong Muslim reconquest of Latin territories in the East and the fall of the last Crusader territory, Acre, in 1291. Eschive d'Ibelin, the Lady of Beirut, and a member of the Ibelin family, one of the most prestigious families of the Latin East who also came to rule the Kingdom of Cyprus, attempted to claim her rights to the Duchy of Athens in 1308, after Beirut was taken by Muslims two decades previously. This was an attempt to regain territorial dominion after conquest. Most families, nevertheless, appear to have stayed in the area: the last king of Jerusalem, Henry II, also ruled as king of Cyprus, and often planned to recover his former territories.

Looking at the aforementioned Ibelin family as a whole, one gets a fairly similar picture. Men stay in the general area of the Eastern Mediterranean, often acting in political office or military duties within the crusader states. Women similarly frequently intermarry with local nobility, while some are also sent to the West as brides.

If anyone knows of any particularly interesting individual cases, I'd be extremely interested to hear, but this is the general picture.

I don't agree with your idea that the Levantine people are a different race - they look about the same as southern Europeans.