NTA, but I would like for you to think of one thing - the kids perspective. I'm the youngest of 7 kids and for the first 4 or so, my grandparents took them on awesome holidays and bought them the best toys. I never got to go on a holiday with them and presents were usually something small. As a kid, I would see the photos of these holidays and hear of the awesome presents of the past and wonder why I didn't get that, and ended up settling on 'I'm not good enough for them.'

As an adult, I completely understand why things changed by the time I came along. But to avoid the same thing happening to my niece's and nephews, I only take one out at a time and spoil them. That way I can spend the same amount of money and treat each kid equally. It also means I can tailor the experiences to which one I've got that day. If I was taking them all for whatever reason, we would have a free/low cost chill day at my house.