Dungarees for toddler

[Eeeeek2]

Bought a pair recently and realised how simple they look to make haha this could be funny I would like a soft stretchy material for all the moving/bending/falling he does plus he is in cloth nappies so hoping to make them with a bit more bum room.

Assuming I need jersey material is that correct? Should I get a pattern or could I give it a good go making one from the dungarees I've got already?

Be careful with dungarees! Sometimes they are easy to make and others they are fiendishly complicated! Lots of people want to start sewing by making them and are nearly always disappointed with the results.
If you look at Kwik Sew's toddler patterns, they have a fair few dungarees (although being a US-owned company, they call them overalls). Kwik Sew patterns tend to be fairly simple, but do watch out for the seam allowances, which vary between patterns.
I wouldn't actually recommend a knit fabric. Designs for toddlers are usually cut to allow for plenty of movement, but measure the pattern pieces across the bum area to make sure there is cloth nappy room- I never found it to be a problem with mine though. You'd be better with a brushed cotton drill, which is stable and headwearing but fairly soft. If you do buy a commercial paper pattern for dungarees, they are usually designed for wovens.
If you can get a copy of the Kwik Sew Sewing For Toddlers book, there are templates to draft all manner of clothes, and I used my copy a lot to make dungaree/overall type things for my brood. I'm not sure if it's still in print, but you can sometimes find it on eBay.
Also, McCall's 6304 is a nice and simple pattern for toddler rompers or overalls.
Sorry, I'm rambling on a bit. Nobody ever lets me talk about sewing patterns, ever!

Argh! Missed your last question, sorry! Yes, get a pattern in a style similar to the ones you like. You might have to compromise a bit if you aren't experienced at sewing, but there are loads of patterns which are designed to be simple to make.

If you want a recommendation, i have made quite a few "Jumpy Rompers" - a pattern from a shop / lady on Etsy called Puperita. They are PDF patterns so you download, stick together with tape then cut out. They sound like they would fit the bill; you can use buttons or snaps to attach around the shoulders. They aren't a particularly hard sew.