I personally wouldn't have hoarded the seats, but without context it's hard to know if this was actually rude, not having been there.
OP: Would it be ok if we grab one of these chairs?
Dad (looking like he wants to fight): No.
Mom (glued to her phone): Our kids are playing video games. May come back and will need somewhere to sit down.
Rude? Definitely.
But what if it were that the family arrived in the packed lounge and stood around for 10 minutes before a small table freed up, and they only had two chairs. They tell their kids to go find something to busy themselves while the parents rest their feet a little. Kids run around a little, come back, hang out in laps then a couple of computers free up. They head over and play a little. Crowd starts to thin out then another two seats free up and they grab them for the kids to sit down and enjoy some snack or drinks. Place suddenly fills up with people again. Enter OP.
OP: Would it be ok if we grab one of these chairs?
Dad (panicked his 13-yo is going to have to sit in his lap again): Oh I'm sorry, these are actually for my kids who are sitting on us, I mean with us.
Mom (frantically tries to wave the kids to come back): They just went to use the computers real quick. CARL! But should be back soon. KARL!
Their sons' names are Carl and Karl, by the way. Twins.
Could they have been more considerate? Maybe. But that scenario wouldn't be rude to me.