Really depends on how much you'll actually ride with the kid and take them on adventures.
If the kid is only riding around the block find a nice used 20" bike on marketplace or craigslist and have it looked over. Don't waste the money on department store, I know all the stories of people who's kids survived on them, I also have witnessed just how bad they really are when people bring them in for repair, they really aren't safe. Pegs on a cheap mongoose are just silly trinkets and an easy way for his friends to break the bike by riding on them with the cheap, thin axle the bike will come with, they're also something extra to hurt themselves on when they crash. If dad is going to show how to legitimately use them the bike will still be 30lbs but will be built heavy duty to use them without failing.
If you're going to ride lots with them, get a decent one. One of the biggest issues is that companies put poor crank lengths on their bikes. The Cannondale trail singlespeed has a 120mm crank, this is the longest a 20" bike should really have, the c-dale trail 20 which is 7sp has 110mm cranks which is better. Giant and Specialized I think in the last two years has moved to putting shorter cranks on to match these, the last time I looked at a trek is was 152mm, which is too long. Although they are more than a department store bike at 330.00 and 450.00 suggested pricing, my kids on decent kids mountain bikes have learned to ride all the local trails. My 6yo over the last year has ridden all the beginner and intermediate trails at 10 different mtb areas near us. He's wanting to ride more and this year will move into the advanced trails. A big part of it is that I love to MTB and the wife does as well, so the kids get brought along about 2x a week in the afternoon or weekends. I wouldn't do it with a walmart bike.