Putting myself first by not having children doesn't mean I'm putting potential children second, as they don't exist.
Having kids (if I wanted them) would not mean that I would strive to put myself first and them second or them first and myself second, it would mean that I would strive for us all to be as happy as possible and for us to be provided for in the best and fullest way possible. As this isn't compatible with the lifestyle I want for myself, I'm not having children.

The outcome of something is generally better when you do it with happiness than without. There is no harm in striving to achieve said happiness if you're not harming others by doing so.
For me, having kids while wanting to be childfree would put my happiness second, because it would mean that at least for a time I would have to let go of things I value very much. I'm not willing to do that and thus childfree.

The objects/people that one spends one's money/time on or emotionally invests in does not automatically determine if said spending is morally incorrect. The relevant factor for rendering spending/emotionally investing "wrong" is existence of a duty to emotionally invest and/or spend.
If you have a pet, you have a duty to take care of it by feeding/taking to the vet/providing a place to sleep/play/socialize etc.
If you have kids, you have a duty to ensure safety and access to healthcare, provide room and board, pay for and help with their schooling, bring them up to be good members of society, develop into independent and healthy beings etc.
Without kids/pets, not spending money/time on them or emotionally investing in them is not wrong.
Investing less in my happiness does not make me less selfish, but investing more helps all in my life because by being happier I want to make them happier as well.