If Maldan was looking to become the future King of Kaldor giving his son to Melderyn would just about guarantee that will never happen - unless by military force with Melderyn as his backer. It make it look like Maldan is making alliances with Melderyn behind the king's back (which he sort of would be).
The only thing - from Silariel's proposal he is not giving his son to Melderyn, but to a knight of Melderyn. Unless this knight is more infuential than his status indicates, aquiring to a Melderyni Knight means little. Squiring him to a Melderyni Earl, a relative of one of the greater houses, or even to a knight with "connections" could be a different thing.
If the knight is not "connected", about all Maldan is gaining perhaps is some contacts for perhaps a mercenary connection, which is minor but could prove to be of a bit of help in the possible upcoming war of sucession.
I think it was not uncommon to squire a son with a knight of another country - but I think you would see this more among higher ranking nobility. On Terra at least, it was a common for both to give a "hostage" as Varlenn messages, but also as a rapport building tactic.
If on Harn this is against their mores, it might be looked at suspiciously - but on Terra IMO it would not be considered suspicious, unless perhaps with a heated rival. And I don't even know if Melderyn would truly be considered a heated rival. Now perchance if Tormau had a son squired with an Earl of Kanday it would be suspicious - but Tormau is already viewed as suspicious.
I think it might be more interesting for Maldan to squire his son with a higher ranking Maelderyni knight. Perhaps even starting up a conversation with the crown that If Maldan were to become King, any thoughts of Chysbia as a vassal to the Kaldoric crown would be put to rest.
And One thing to think about as well - it appears Harn has many batchelor knights without formal employment - per battlelust, some of these are available for a feudal muster (though I would think they would have to be paid as mercenaries, not serving at least a portion of their time for free).
These batchelor knights owe no feudal service to the crown - this is likley where the biggest source of mercenary knights come from. If Maldan were able to get a bit of influence in Melderyn, this could be a good source of mercenaries. And it would not be an act of war by Melderyn - these are mercenary knights, not agents of the crown. Now if Melderyn openly financed them, that would be another matter.