I also don't think age has anything to do with thanking someone, if you are given a gift, no matter what it is, you say "thank you".
ITA. My only issue in my family now is my brother and SIL. I really don't want to give them anything, but feel that I'm obligated to. The problem is I do want to buy gifts for my other family members and feel that their gifts must be equal. Normally, I would have no issue with giving without it being reciprocated (I actually don't like receiving gifts), but it's just their attitude that makes me resent giving to them. They just have this sense that other people are supposed to give them things, yet they give no thought to giving to others or even acting like they appreciate what's given to them.
For example, SIL attended both my and my sister's baby showers and did not give either of us a gift. Fine, but when she was pregnant they did not purchase anything for their baby because they expected that everything would be provided to them at their baby shower. Also, they expected a certain money amount from everyone who attended their wedding, but when we went to a cousin's wedding SIL actually had the audacity to ask my mother if they were "supposed" to give a gift...

SIL also tends to turn up her nose at anything you give her and is very difficult to buy for anyway. Now that they have a child this is even causing problems with giving to the niece. Last year my sister gave the baby a certain toy that she gave every other niece/nephew for their first Christmas, so this was a special/meaningful gift. Instead of just saying "Thanks", SIL ruined my sister's joy by telling her she needed to return it and get her something else because "Santa" was already bringing that toy. WTH? The baby was only 4mos and my sister was not going to be at their house Christmas morning, so why not just act grateful and return the one you bought.

My brother and SIL just sold their house and are planning to move across the country. I will send gifts to my niece as long as I feel they are appreciated by the child, but I have no intention of sending gifts for the adults. I guess I just give to them at Christmas, so I don't feel awkward giving everyone else gifts while they're sitting there. But honestly, they don't feel awkward "forgetting" to bring gifts to all the nieces' and nephew's birthday parties so I don't really know why I should feel bad. ugh.
This has mostly just turned into a vent, but related to the OP I would say something to the parents about not being acknowledged and try to find out whether the kids actually apprecitate the gifts or not. Then I would consider not giving to them any more.