Why do you think the fat bike is the way forward for you?
I don't see anything but a really draggy ride there. The tires and tubes cost a fortune, the wheels hard to replace if anything goes wrong. I'd recommend 'normal' size mountain bike or hybrid wheels and tires. If you're riding on smooth surfaces you don't need suspension, so there's a lot of bikes out there to better suit you than a fat bike.

 

I assumed fat bike with its steel frame and 4" wide tire would easily take my weight. What other bikes are you taking about that are under $400?

 

I was your weight, but considerably shorter at 5'9". If you're not aggressively mountain biking like jumping, drops, etc. you don't really need a tank of a bike. I wouldn't recommend a Wal-Mart bike, but a nice rigid bike with gears, properly fitted by the shop, is night and day better than a cheap Wal-Mart bike.

With regards to the baby carrier, you may want to reconsider that and get a trailer. I say this because the baby carrier has low weight limits, it's a pain to put on the bike and keep the kiddo together while keeping the bike upright. Additionally, putting the kiddo so high on the bike could increase the possibility of injury if you were to crash. The trailers attach to the chain stay or seatpost. There's a much lower chance of the kids getting hurt. Lastly, the trailer has a longer usable life span in that the kids can ride on it much longer. My 12 and 13 year olds ride their bikes, but the 2 year old and 5 year old just can keep up, so they ride in the trailer. It's a great workout with 80 lbs of kid and trailer behind you!

 

Hi. I am a little bit taller and bigger. I suggest a Trek Marlin 5/6/7. The Trek Marlin 5 run $549 new - https://www.trekbikes.com/us/en_US/bikes/mountain-bikes/cross-country-mountain-bikes/marlin/marlin-5/p/23134/. I had a Marlin 6 in XXL I sold last month for $450. You might be able to find one used in your area.

A brand like Trek/Specialized/Giant will generally be a better quality bike than the Mongoose.

A Giant ATX 3 disk is $480 retail https://www.giant-bicycles.com/us/bikes-atx.

A Specialized Rockhopper is $500 - https://www.specialized.com/us/en/rockhopper-29/p/184115?color=291598-184115&searchText=91520-7802

The 29in tires will roll better and is better suited for your height. The costs of parts will be cheaper and the extra cost will give you a more durable and better bike. Check with your local bike shop for used bikes.

I also think finding a child trailer would be better than a child seat. Those pop up on craigslist and other sites. I hope this helps.

 

I aswell looking to buy some fat bike and imo Mongoose Dolomite is OP > but i have few problems with it
1. i'm 6.8 and 350 lbs - not sure if it will fit my hight (sure it can handle the weight)
2. it not avalible in my country and due to the pandemic, i cannot deliver it right now
What i will do for now > buy Trek marlin 7 29 2020 and mod wheel\seat
but hopefully later this year i will order aswell Mongoose dolomite or Mongoose Argus Sport

From my little research of webb the dolomite seems like the way to go with 6.2"
(sadly delivery will cost me 200$ +- but i think it's worth it with my 6.8" )

 

I aswell looking to buy some fat bike and imo Mongoose Dolomite is OP > but i have few problems with it
1. i'm 6.8 and 350 lbs - not sure if it will fit my hight (sure it can handle the weight)
2. it not avalible in my country and due to the pandemic, i cannot deliver it right now
What i will do for now > buy Trek marlin 7 29 2020 and mod wheel\seat
but hopefully later this year i will order aswell Mongoose dolomite or Mongoose Argus Sport

From my little research of webb the dolomite seems like the way to go with 6.2"
(sadly delivery will cost me 200$ +- but i think it's worth it with my 6.8" )

Where are you located? Pole make the Taiga which is a big fat bike. I ride a Pole Taival at 6'5" and it is a good fit and I think the Taiga has same/similar dimensions