If I marry a foreigner and intend to have 2 kids, would it be better to buy a four room resale flat or apply for a four room BTO with my parents?

As someone married to a foreigner and father of two kids, I would cast my vote for four room resale flat. Because that's what I did. My living room is practically my kids' playpen and I cannot imagine how we are gonna survive with limited space in a BTO unit

It is not about "better" per se but your eligibility and affordability.
You can only buy BTO if your spouse is a SPR and you are a SG citizen if not forget about it not eligible.
For resale, your spouse must be at least on LTVP pass but cannot be a co-owner but just a legal occupier status in the paperwork. Any other pass like EP/WP/Spass not applicable.
Of course if you are above 35 years and can afford it can just buy solo. Your spouse can form the family nucleus if needed.
If you are not SG citizen but SPR you cannot buy a resale HDB until after 3-5 years SPR issued.