Of the 4 national carriers, it appears that the cheapest family plan starts at $59.99/mo with AT&T (3 lines maximum only) or T-Mobile Even More Family Talk (up to 5 lines). I'm unsure if the T-Mobile Even More Plus Family plans are still available since they no longer appear on the website, but they used to start at $49.99/mo. Any HoFos in the know?
When compared against the $69.99/mo price point for a family plan (minimum 2 lines) offered by Verizon and Sprint with 2-year contracts, a few non-contract options emerged as cost-effective alternatives:
MetroPCS Family Plan (2 lines): starting at $70/mo for unlimited talk/text/metroWEB (not sure if a 7-year-old should have unlimited text or metroWEB)2 Cricket Basic Plans: $35+$35 =$70/mo for unlimited talk/text (again not sure if a 7-year-old should have unlimited text)2 Cricket PAYGO Basic Plans: $1/day unlimited talk. Assuming 2 lines for 31 days, that's $31 per line for $62/mo (depending on the length of the month).
For unlimited carrier mobile-to-mobile talk for 2 lines, it looks like the $59.99/mo AT&T Family Plan (3-lines max) or T-Mobile Even More Family Talk (up to 5 lines) can't be beat, except perhaps by a $49.99/mo T-Mobile Even More Plus Family Plan. However after fees + taxes are factored in, it looks like the Cricket PAYgo Basic Plan is competitive (assuming decent Cricket coverage for the OP's situation).
HTH...