Best sandy beaches on the western coast, anywhere from Tuscany north?

You are correct CT is out (and would be unbearable in summer). I can't say what they will be like in July (other than reserve a spot!), but Santa Margarita Ligure and Camogli would be nice. I have heard good things about Sestri Levante with children.
Those northern Tuscan beaches (Viareggio, Forte de Marmi) are the formal, chairs lined up type, and I just don't find them appealing. Of course in summer, the others will probably be like that as well. You could try searching for a "natural" beach. There is a large coastal park in Tuscany called la Maremma, which might be worth looking into.

Thanks for your comments. Is it even worth heading to the Italian Riveria during this time? We were going to go in April but everyone told us it would be too cold to swim so decided to wait.

Are we better off just sticking to the lakes in northern Italy?

That's a huge stretch of the Italian coast that is hundreds of km long, there are thousands of beaches that could meet your requirements. Where exactly are you going?

Liguria: There are sand beaches in nearly every town from Genoa all the way to Lerici. The Cinque Terre are a bit rocky, but Monterosso has an actual sand beach. The water gets deep quickly however, therefore be careful.

I used to stay in Forte dei Marmi (Versilia), about 1 hr from Monterosso.
From Forte dei Marmi to Viareggio, the beaches are similar. Very large, Florida type, mostly organized with beach concessions that can rent you umbrellas, chairs, changing cabins, bars, access to swimming pools. Those are very sandy and definitely ideal for families with children. If you have a car, you can reach all easily. By train maybe Viareggio is easier.

South of Viareggio, between Viareggio and Torre del Lago, there is also a major Pine forest along the coast with a less crowded beach called La Lecciona.

There are thousands more, and the best coastal spots in Tuscany are the islands and southern Tuscany (south of Piombino). I'm not going to mention any of that because that may not be in your itinerary.

If you go in July, I would probably find time for a beach. Anyplace is good, but at the Cinque Terre only Monterosso would have a beach. Nearby there is also Levanto, which has a beach.

Every meter of the beautiful beaches of Italy will be peopled in July. I suggest getting a hotel with beach rights and pretend you're Italian. The coast north of Viareggio is entirely flat child-friendly beach, yet you can easily make excursions as the autostrada is nearby. One small coastal town I like is Marina di Massa (not Massa Marittima). It has a nice beach, a pier, it's not snobby, has an autostrada entrance and is not excessively dense. You can visit Carrara, Lucca, Pisa, Lerici with less than an hour's drive. There is also an excursion boat to the Cinque Terre stopping at Portovenere and Monterosso ( last summer's times at the bottom here: http://www.apathtolunch.com/2014/06/complete-cinque-terre-portovenere-la.html ).

Ha, yes I guess it is a long stretch. We are flying into MXP (Milan). Was thinking of going straight to Lake Como or Lake Maggiore, spending a few days there, then making our way to Tuscany and stopping at a beach town on the way.

I'm not sure that makes sense logistically? I guess if we were driving it would but maybe not if we're taking the train. We def need a car in Tuscany.

My sense is perhaps we should choose one or the other - Ligurian coast or lake. I personally like the mountain-to-sea look. I love the steep cliffs and aquamarine waters. This is why CQ appeals to me. But perhaps we can find the same thing at Lake Como or Lake Maggiore?

Late July I would say reconsider, but early July should be okay. I say go for it!

I don't think the lake beaches are as good as the sea, but kids will like anything. The Cinque Terre are always crowded, but elsewhere it won't be too bad until late July and August.

If you like the mountains to sea look, then along the way the Ligurian coast provides plenty of that. Monterosso or Levanto are good and easy to reach from the freeway (A12 exit: Levanto) if you drive.

If you go from lake Maggiore to Tuscany, the coastal route makes sense. Just take the A26 freeway from Stresa and follow it all the way to Genova (Genoa). Just before Genova you merge onto the A10 and then follow directions to Livorno (at Genova the freeway becomes A12).

If you go to Lake Como, then it's a longer detour.

Sandy beach I enjoyed - Levanto.
We stayed there on the RS Village Italy tour and took the train to the CT.

Rddysum, Roberto's advice, as usual, is spot on.

Thanks! Monterosso is part of CQ, right? Won't be mobbed first week of July?

Yes. Monterosso is part of the Cinque Terre. There will be crowds in July.
Levanto is not technically part of the Cinque Terre, but only 4 min. from Monterosso by train.

I used to go to Moneglia a lot, which I loved because it's very quaint and full of palms

(A12 freeway Exit: Sestri Levante)