I haven’t seen it recommended yet (unless I missed it), but Pello bikes are a great company. They are light and have a better geometry for kids. They are based out of Richmond, VA, (which is where I live), so I was able to pick mine up at their warehouse. I took my middle daughter with me to pick up her older sister’s bike and she was able to test ride a 16" wheel Revo model. She was much more comfortable on it than her 16" Giant bike at home and really loved that it had a hand brake (coaster brakes are required by law on all bikes under 20" wheel size). The bikes are lighter than our Giant kids’ bikes at home, and they seem to fit our kids a lot better. They just came out with a 24" wheel model. So now they have models ranging from a 12" wheel balance bike to a 24" mountain bike with disc brakes and a front shock. Limited color options though, but that’s an effort to keep their prices down.
Choosing a bike for a kid is so boring and hard. There are a lot of bicycles available for the kids. Some of the bikes are good. you need carefully searching them. Better to depend on online bicycle reviews site to get a perfect kids bike. you can check here.
I recommend RoyalBaby BMX Freestyle bike for the toddler.
A bit cheaper than Frog and Islabikes but the Ridgeback MX14/MX16 are worth checking out. We got the MX14 for out two year for his their birthday, he was able to move from his Stryder balance bike to the MX14 the day before his third birthday. If a two year old (just!) can manage it I’m sure your 3/4 year old will manage just fine. They’re nice and sturdy and not to heavy.
If you’ve got lots of free cash, sure get your kids expensive bikes - maybe you’ll be lucky and they’ll use them a lot. I don’t mean to come across critical, but it’s totally legit to get cheap/crappy bikes too. They don’t use bikes nearly like how we do, even if they’re keen, I think the price is hard to justify.
I see what you’re saying. It may be confirmation bias but I have seen kids struggling on cheaper heavier bikes.
The other factor to consider is that the more expensive ones hold their value. After 2 years of use on my boy’s previous frog bike we only lost around £80 from what we paid. That’s decent value for the use I think.
The same does not hold true around here. If you search up craigslist there are no end of formerly 3-500 dollar treks for 50. This is why I chose to build a bike that would grow with my son.
Here in UK at least isla and frog bikes especially really hold value well, especially if buy 2nd hand then likely sell it on for same price in few years time if look after it.
In terms of kids not using bikes much - I find my pair enjoy riding the much lighter bikes mich more…so ride them lot more.
Saying that…my 6 yr old is nagging my life out to try cyclocross after I let slip I saw lad his size at local race. So this summer he’s going to race the local summer cx series (excellent as I wanted to race it anyway!)
The kids cross racing is really fun. They run the U8s and U10s together at my local series and you wouldn’t believe the speed of some of the little kids on their 14" wheel bikes!
Be careful though - it could cost you a new bike! My daughter has ridden three of them this winter and is now pestering me to get her a Luath. 
There’s some serious kit on the start lines at 'cross (makes sense I guess - enthusiast parents), but so long as you buy secondhand and look after it, it won’t be expensive in the long run.
Why not just buy a $500 Trek for $50?
We did that too when they were younger. They each had a 24” trek, or Jamis. The return diminishes when they get a bit older. Consider how long a new bike is going to fit say, and 11 year old. They start at 35lbs and 40” tall and by the time they’re 13 they’re 100 lbs and 5 ‘ tall. It’s just easier to size up to a 26 ASAP unless they’re already racing because hey basically ride up and down the street. My kids all started riding more seriously around the 13 year time frame.
Great Information
Regards
Umair
For really young kids a balance bike is brilliant. We did this with our 3rd daughter and when it was time to put her on a real bike she went across the front lawn once, started pedalling and was then riding. It was virtually instant.
We used Isla bikes for all our kids. They are excellent, and when you finish with them you sell then on eBay…