I understand where you are coming from BUT! If you took an angel and their request was a car seat you are sort of morally obligated to get that car seat.
I can't tell you how awful it feels to go to sign up for this program and consider carefully what is put on that list and tell specific items needed or wanted. Them to go to the Salvation Army to pick up your kids gifts and find it's clothes instead of the bike they were really really wanting that you could never afford. They have closets full of clothes since everyone gives you tons of hand me downs and they get more clothes instead of the bike...that was so hard. I had to go pawn my wedding rings and go buy a bike just a few days before Christmas due to this.
Plus the clothes were on clearance and the wrong size so we ended up giving them to a local charity. We couldn't use them...
So if you really don't want to spend the bucks to get them a car seat you might see if the angel tree still has kids on it then exchange your little ones name for one that has the gifts on it that you'd really like to give.
I know your heart is in the right place, this is not something that everyone even thinks of doing. I appreciate what you are doing. Just please get them the items they listed, that's what they are really really hoping for.
The fire department and often the local Health Department will get car seats for kids BUT they have to have the funds allotted for that and we all know how many programs for the poor have been cut and cut and cut and cut over the last 4 years. Now even more of them are being cut out.