Is it rude?

So my husbands cousin has triplets who we haven't even seen but twice their whole life. They just turned 3 & their birthday party is tomorrow. Myself & my son(9 months old) were invited to the party...husband is deployed. We haven't gotten them a gift. I have absolutely no idea what they even like or need since they own everything lol. Is it rude if we don't get them a gift?
Side note I don't expect them to get my son anything for his birthday as we aren't having a huge party.

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