thanks to you all for the motivation!! next project I have a few ideas,maybe making a frontdoor sign with my palm router or a fancy tray for serving dinner!
follow up question.If i made a rabeet on the upper and lower panels and just glued there the lateral panels,would that be strong enough to hold all the boxes with books and toys? (the middle panel maybe I would still do with pocket screws,or maybe glue it in a groove)
Depends a little on how thick the material is. If it's 18mm (3/4) then rabbets and dados will make a difference, but for the center horizontal board, your dados will have to be shallow since they will be on both sides in the same place. You want about half the material left, so your dados will only be about 4mm deep. But Top, bottom and side dados and rabbets will be closer to 6 - 8mm deep. I'd put the rabbets on the short, side pieces.
Glue: On edges of ply, I always apply glue, let it dry, then apply another coat in both the dado and the piece you're inserting. You could use pocket screws, but their holes are hard to fill and make look nice. You can't sand too much or you'll go through the very thin finished layer.
Glue and clamping, I think is your best bet. You will have some squeeze out on the glue, but use a chisel to scrape it off. Since you are painting it, you'll cover up any minor errors.
If using sheet ply, I would cut a chunk a little wider than double the depth of the top and bottom pieces and cut the dados straight across using a good T Square. Do the same with the rabbets on the end pieces, then cut that in half.
I'd definitely recommend you make a relatively simple Perfect Fit Dado Jig. Getting dados just right is much easier. If they are off, nothing will fit together. There are other techniques if you have the tools for it, but for dados and grooves, having the perfect fit jig is the best way to go in my thinking. Here's a drawing of a simple one.
You fit the piece that goes into the dado in this edgewise, snug the movable piece against it, lock it down, then use a small straight or mortising bit with a bearing and make two passes and you have your dado. Easy to make this jig. Use the edges of the sheet goods to make the inside track for the bearing to ride against. These are usually very straight. Be careful to make sure the fied side pieces are precisely 90 degrees so the dados line up perfectly. I would make this about 32 inches wide so I could cut any width up to 24 inches wide.
Here's the bit I'd use, a 3/8ths mortising bit because mortising bits produce a very flat bottom cut, which means better glue grip. This has a half inch shank, but you can get one in 3/8ths with a quarter inch shank pretty easily. Amazon has one for pretty cheap. Great for mortising for hinges as well. Make multiple passes, about 3 mm per pass.