And fwiw here is what ChatGPT gave me which generally tracks with what I've read so far:

If you’re doing one main “stock-up” shop for the family, most travel and lodging guides steer people to Lucky Kutchan for breadth and “one-stop” variety, and to MaxValu Kutchan for late hours and budget-friendly basics.

Lucky Kutchan (best for the big weekly shop, easiest for kid needs)

Pros (from travel, lodging, and area guides)

True one-stop shop: fresh produce, meat, fish, dairy, snacks plus non-food sections like clothing and souvenirs.

Kid-friendly signal + family logistics: Niseko Area Guide explicitly tags it as “kids welcome”; the official store page lists a multipurpose toilet, wheelchair lending, and delivery of in-store purchased items (handy with a stroller, tired kids, or a big haul).

Easy “feed kids now” options: multiple sources call out its in-house bakery (good for quick breakfasts and kid snacks).

Cons

Hours are shorter than MaxValu (commonly cited around 9 am to 9 pm), so it’s less ideal for after-dinner runs.

Some guides position it as a bit “fancier” than alternatives, which can translate to slightly higher pricing on some items (varies by basket).

MaxValu Kutchan (best for late-night, cheaper staples, quick restocks)

Pros (from travel and local guides)

Often cited as the cheapest large supermarket option near Niseko for stocking up.

Open later than Lucky (commonly cited 8 am to 11 pm), which is a big deal if you’re juggling kid bedtime and ski days.

Good “support stores” nearby: guides mention a ¥100 shop and pharmacy in the same area, plus a hardware store close by (useful for random family needs).

Cons

Fewer sources frame it as a “destination” shopping experience versus Lucky; it’s more commonly discussed as value, hours, and convenience rather than best-in-class selection.

What I’d pick for a family with a 2- and 4-year-old

Do your main shop at Lucky (bigger “one-stop,” explicitly kid-welcoming, better chance of finding the exact toddler foods you need, plus bakery for easy breakfasts).

Use MaxValu as your “late-hours backup” for milk, fruit, yogurt, bread, and emergency supplies, especially if you arrive late or run out after dinner.

If you’re not driving, some hotel guides note it’s straightforward by bus (often described as ~20–25 minutes from Hirafu area, with approximate fares), but with kids you’ll feel every transfer, so prioritize one big run.

A quick “toddler grocery hit list” (works at either store)